Example apps: Don't use runtime IO config API if CBs are hardcoded

Multiple example applications still use mbedtls_ssl_set_bio() even
if the I/O callbacks are hardcoded. This commit fixes this.
diff --git a/programs/ssl/dtls_server.c b/programs/ssl/dtls_server.c
index f2dcd2f..ad3a70a 100644
--- a/programs/ssl/dtls_server.c
+++ b/programs/ssl/dtls_server.c
@@ -305,8 +305,15 @@
         goto exit;
     }
 
-    mbedtls_ssl_set_bio( &ssl, &client_fd,
-                         mbedtls_net_send, mbedtls_net_recv, mbedtls_net_recv_timeout );
+#if !defined(MBEDTLS_SSL_CONF_RECV) &&          \
+    !defined(MBEDTLS_SSL_CONF_SEND) &&          \
+    !defined(MBEDTLS_SSL_CONF_RECV_TIMEOUT)
+        mbedtls_ssl_set_bio( &ssl, &client_fd,
+                             mbedtls_net_send, mbedtls_net_recv,
+                             mbedtls_net_recv_timeout );
+#else
+        mbedtls_ssl_set_bio_ctx( &ssl, &client_fd );
+#endif
 
     printf( " ok\n" );