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 |

programs/ssl/ssl_client2.c

@@ -1764,7 +1764,12 @@ int main( int argc, char *argv[] )
         }
 #endif
 
+#if !defined(MBEDTLS_SSL_CONF_RNG)
     mbedtls_ssl_conf_rng( &conf, mbedtls_ctr_drbg_random, &ctr_drbg );
+#else
+    mbedtls_ssl_conf_rng_ctx( &conf, &ctr_drbg );
+#endif
+
     mbedtls_ssl_conf_dbg( &conf, my_debug, stdout );
 
 #if !defined(MBEDTLS_SSL_CONF_READ_TIMEOUT)