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/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 74c9f1a..52835b4 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1289,6 +1289,27 @@
 #endif /* MBEDTLS_SSL_CONF_ANTI_REPLAY */
 #endif /* MBEDTLS_SSL_DTLS_ANTI_REPLAY */
 
+typedef int (*mbedtls_frng_t)( void*, unsigned char*, size_t );
+
+#if !defined(MBEDTLS_SSL_CONF_RNG)
+static inline mbedtls_frng_t mbedtls_ssl_conf_get_frng(
+    mbedtls_ssl_config const *conf )
+{
+    return( conf->f_rng );
+}
+#else /* !MBEDTLS_SSL_CONF_RNG */
+
+#define mbedtls_ssl_conf_rng_func MBEDTLS_SSL_CONF_RNG
+extern int mbedtls_ssl_conf_rng_func( void*, unsigned char*, size_t );
+
+static inline mbedtls_frng_t mbedtls_ssl_conf_get_frng(
+    mbedtls_ssl_config const *conf )
+{
+    ((void) conf);
+    return ((mbedtls_frng_t*) mbedtls_ssl_conf_rng_func);
+}
+#endif /* MBEDTLS_SSL_CONF_RNG */
+
 #if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
 static inline unsigned int mbedtls_ssl_conf_get_ems(
     mbedtls_ssl_config const *conf )