Allow compile-time configuration of I/O function pointers

Introduce the compile-time options
- MBEDTLS_SSL_CONF_RECV
- MBEDTLS_SSL_CONF_SEND
- MBEDTLS_SSL_CONF_RECV_TIMEOUT
which can be used to configure the callbacks for the underlying
transport at compile-time.

Code-size impact:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` before | 23471 | 24077 | 27045 |
| `libmbedtls.a` before | 23379 | 23981 | 26941 |
| gain in Bytes | 92 | 96 | 104 |
diff --git a/programs/ssl/mini_client.c b/programs/ssl/mini_client.c
index 18e11d0..4231c0c 100644
--- a/programs/ssl/mini_client.c
+++ b/programs/ssl/mini_client.c
@@ -269,7 +269,13 @@
         goto exit;
     }
 
+#if !defined(MBEDTLS_SSL_CONF_RECV) && \
+    !defined(MBEDTLS_SSL_CONF_SEND) && \
+    !defined(MBEDTLS_SSL_CONF_RECV_TIMEOUT)
     mbedtls_ssl_set_bio( &ssl, &server_fd, mbedtls_net_send, mbedtls_net_recv, NULL );
+#else
+     mbedtls_ssl_set_bio_ctx( &ssl, &server_fd );
+#endif
 
     if( mbedtls_ssl_handshake( &ssl ) != 0 )
     {