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/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 378db24..20d2006 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1085,6 +1085,21 @@
  * be fixed at compile time via one of MBEDTLS_SSL_SSL_CONF_XXX.
  */
 
+#if !defined(MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION)
+static inline unsigned int mbedtls_ssl_conf_get_allow_legacy_renegotiation(
+    mbedtls_ssl_config  const *conf )
+{
+    return( conf->allow_legacy_renegotiation );
+}
+#else /* !MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION */
+static inline unsigned int mbedtls_ssl_conf_get_allow_legacy_renegotiation(
+    mbedtls_ssl_config  const *conf )
+{
+    ((void) conf);
+    return( MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION );
+}
+#endif /* MBEDTLS_SSL_CONF_ALLOW_LEGACY_RENEGOTIATION */
+
 #if !defined(MBEDTLS_SSL_CONF_AUTHMODE)
 static inline int mbedtls_ssl_conf_get_authmode(
     mbedtls_ssl_config  const *conf )