Allow compile-time configuration of legacy renegotiation

Introduces MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION
allowing to configure enforcing secure renegotiation at
compile-time.

Impact on code-size:

|  | GCC | ARMC5 | ARMC6 |
| --- | --- | --- | --- |
| `libmbedtls.a` after  | 23379 | 23929 | 27727 |
| `libmbedtls.a` before | 23307 | 23865 | 27615 |
| gain in Bytes | 72 | 64 | 112 |
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index dc5542e..879fcf4 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -301,6 +301,12 @@
 #define USAGE_AUTH_MODE ""
 #endif
 
+#if !defined(MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION)
+#define USAGE_ALLOW_LEGACY_RENEGO    "    allow_legacy=%%d     default: (library default: no)\n"
+#else
+#define USAGE_ALLOW_LEGACY_RENEGO ""
+#endif
+
 #define USAGE \
     "\n usage: ssl_client2 param=<>...\n"                   \
     "\n acceptable parameters:\n"                           \
@@ -332,7 +338,7 @@
     USAGE_ECJPAKE                                           \
     USAGE_ECRESTART                                         \
     "\n"                                                    \
-    "    allow_legacy=%%d     default: (library default: no)\n"   \
+    USAGE_ALLOW_LEGACY_RENEGO                               \
     USAGE_RENEGO                                            \
     "    exchanges=%%d        default: 1\n"                 \
     "    reconnect=%%d        number of reconnections using session resumption\n" \
@@ -987,6 +993,7 @@
                 MBEDTLS_SSL_RENEGOTIATION_ENABLED :
                 MBEDTLS_SSL_RENEGOTIATION_DISABLED;
         }
+#if !defined(MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION)
         else if( strcmp( p, "allow_legacy" ) == 0 )
         {
             switch( atoi( q ) )
@@ -1003,6 +1010,7 @@
                 default: goto usage;
             }
         }
+#endif /* !MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION */
         else if( strcmp( p, "renegotiate" ) == 0 )
         {
             opt.renegotiate = atoi( q );
@@ -1771,8 +1779,10 @@
         mbedtls_ssl_conf_arc4_support( &conf, opt.arc4 );
 #endif
 
+#if !defined(MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION)
     if( opt.allow_legacy != DFL_ALLOW_LEGACY )
         mbedtls_ssl_conf_legacy_renegotiation( &conf, opt.allow_legacy );
+#endif /* !MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION */
 #if defined(MBEDTLS_SSL_RENEGOTIATION)
     mbedtls_ssl_conf_renegotiation( &conf, opt.renegotiation );
 #endif