Allow compile-time configuration of PRNG in SSL module

Introduces MBEDTLS_SSL_CONF_RNG to allow configuring the
RNG to be used by the SSL module at compile-time.

Impact on code-size:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` before | 23535 | 24089 | 27103 |
| `libmbedtls.a` after | 23471 | 24077 | 27045 |
| gain in Bytes | 64 | 12 | 58 |
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 2749389..a050adb 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -394,7 +394,8 @@
 
         ret = mbedtls_ecjpake_write_round_one( &ssl->handshake->ecjpake_ctx,
                                         p + 2, end - p - 2, &kkpp_len,
-                                        ssl->conf->f_rng, ssl->conf->p_rng );
+                                        mbedtls_ssl_conf_get_frng( ssl->conf ),
+                                        ssl->conf->p_rng );
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1 , "mbedtls_ecjpake_write_round_one", ret );
@@ -751,14 +752,20 @@
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, current time: %lu", t ) );
 #else
-    if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng, p, 4 ) ) != 0 )
+    if( ( ret = mbedtls_ssl_conf_get_frng( ssl->conf )
+          ( ssl->conf->p_rng, p, 4 ) ) != 0 )
+    {
         return( ret );
+    }
 
     p += 4;
 #endif /* MBEDTLS_HAVE_TIME */
 
-    if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng, p, 28 ) ) != 0 )
+    if( ( ret = mbedtls_ssl_conf_get_frng( ssl->conf )
+          ( ssl->conf->p_rng, p, 28 ) ) != 0 )
+    {
         return( ret );
+    }
 
     return( 0 );
 }
@@ -822,7 +829,7 @@
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> write client hello" ) );
 
-    if( ssl->conf->f_rng == NULL )
+    if( mbedtls_ssl_conf_get_frng( ssl->conf ) == NULL )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "no RNG provided") );
         return( MBEDTLS_ERR_SSL_NO_RNG );
@@ -908,7 +915,8 @@
         ssl->session_negotiate->ticket != NULL &&
         ssl->session_negotiate->ticket_len != 0 )
     {
-        ret = ssl->conf->f_rng( ssl->conf->p_rng, ssl->session_negotiate->id, 32 );
+        ret = mbedtls_ssl_conf_get_frng( ssl->conf )
+            ( ssl->conf->p_rng, ssl->session_negotiate->id, 32 );
 
         if( ret != 0 )
             return( ret );
@@ -2333,7 +2341,8 @@
     mbedtls_ssl_write_version( ssl->conf->max_major_ver, ssl->conf->max_minor_ver,
                        ssl->conf->transport, p );
 
-    if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng, p + 2, 46 ) ) != 0 )
+    if( ( ret = mbedtls_ssl_conf_get_frng( ssl->conf )
+          ( ssl->conf->p_rng, p + 2, 46 ) ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, "f_rng", ret );
         return( ret );
@@ -2382,7 +2391,8 @@
                             p, ssl->handshake->pmslen,
                             ssl->out_msg + offset + len_bytes, olen,
                             MBEDTLS_SSL_OUT_CONTENT_LEN - offset - len_bytes,
-                            ssl->conf->f_rng, ssl->conf->p_rng ) ) != 0 )
+                            mbedtls_ssl_conf_get_frng( ssl->conf ),
+                            ssl->conf->p_rng ) ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_rsa_pkcs1_encrypt", ret );
         goto cleanup;
@@ -3155,7 +3165,8 @@
         ret = mbedtls_dhm_make_public( &ssl->handshake->dhm_ctx,
                                 (int) mbedtls_mpi_size( &ssl->handshake->dhm_ctx.P ),
                                &ssl->out_msg[i], n,
-                                ssl->conf->f_rng, ssl->conf->p_rng );
+                                mbedtls_ssl_conf_get_frng( ssl->conf ),
+                                ssl->conf->p_rng );
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_dhm_make_public", ret );
@@ -3169,7 +3180,8 @@
                                       ssl->handshake->premaster,
                                       MBEDTLS_PREMASTER_SIZE,
                                      &ssl->handshake->pmslen,
-                                      ssl->conf->f_rng, ssl->conf->p_rng ) ) != 0 )
+                                      mbedtls_ssl_conf_get_frng( ssl->conf ),
+                                      ssl->conf->p_rng ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_dhm_calc_secret", ret );
             return( ret );
@@ -3206,7 +3218,8 @@
         ret = mbedtls_ecdh_make_public( &ssl->handshake->ecdh_ctx,
                                 &n,
                                 &ssl->out_msg[i], 1000,
-                                ssl->conf->f_rng, ssl->conf->p_rng );
+                                mbedtls_ssl_conf_get_frng( ssl->conf ),
+                                ssl->conf->p_rng );
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecdh_make_public", ret );
@@ -3235,7 +3248,8 @@
                                       &ssl->handshake->pmslen,
                                        ssl->handshake->premaster,
                                        MBEDTLS_MPI_MAX_SIZE,
-                                       ssl->conf->f_rng, ssl->conf->p_rng ) ) != 0 )
+                                       mbedtls_ssl_conf_get_frng( ssl->conf ),
+                                       ssl->conf->p_rng ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecdh_calc_secret", ret );
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)
@@ -3317,7 +3331,8 @@
             ret = mbedtls_dhm_make_public( &ssl->handshake->dhm_ctx,
                     (int) mbedtls_mpi_size( &ssl->handshake->dhm_ctx.P ),
                     &ssl->out_msg[i], n,
-                    ssl->conf->f_rng, ssl->conf->p_rng );
+                    mbedtls_ssl_conf_get_frng( ssl->conf ),
+                    ssl->conf->p_rng );
             if( ret != 0 )
             {
                 MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_dhm_make_public", ret );
@@ -3334,7 +3349,8 @@
              */
             ret = mbedtls_ecdh_make_public( &ssl->handshake->ecdh_ctx, &n,
                     &ssl->out_msg[i], MBEDTLS_SSL_OUT_CONTENT_LEN - i,
-                    ssl->conf->f_rng, ssl->conf->p_rng );
+                    mbedtls_ssl_conf_get_frng( ssl->conf ),
+                    ssl->conf->p_rng );
             if( ret != 0 )
             {
                 MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecdh_make_public", ret );
@@ -3376,7 +3392,8 @@
 
         ret = mbedtls_ecjpake_write_round_two( &ssl->handshake->ecjpake_ctx,
                 ssl->out_msg + i, MBEDTLS_SSL_OUT_CONTENT_LEN - i, &n,
-                ssl->conf->f_rng, ssl->conf->p_rng );
+                mbedtls_ssl_conf_get_frng( ssl->conf ),
+                ssl->conf->p_rng );
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecjpake_write_round_two", ret );
@@ -3385,7 +3402,8 @@
 
         ret = mbedtls_ecjpake_derive_secret( &ssl->handshake->ecjpake_ctx,
                 ssl->handshake->premaster, 32, &ssl->handshake->pmslen,
-                ssl->conf->f_rng, ssl->conf->p_rng );
+                mbedtls_ssl_conf_get_frng( ssl->conf ),
+                ssl->conf->p_rng );
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecjpake_derive_secret", ret );
@@ -3583,7 +3601,8 @@
     if( ( ret = mbedtls_pk_sign_restartable( mbedtls_ssl_own_key( ssl ),
                          md_alg, hash_start, hashlen,
                          ssl->out_msg + 6 + offset, &n,
-                         ssl->conf->f_rng, ssl->conf->p_rng, rs_ctx ) ) != 0 )
+                         mbedtls_ssl_conf_get_frng( ssl->conf ),
+                         ssl->conf->p_rng, rs_ctx ) ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_pk_sign", ret );
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)