Add support for RM2 with STM32F (429, 746)

src/net_builtin.h

@@ -26,6 +26,7 @@ enum {
   MG_TCPIP_EV_WIFI_SCAN_RESULT, // Wi-Fi scan results             struct mg_wifi_scan_bss_data *
   MG_TCPIP_EV_WIFI_SCAN_END,    // Wi-Fi scan has finished        NULL
   MG_TCPIP_EV_WIFI_CONNECT_ERR, // Wi-Fi connect has failed       driver and chip specific
+  MG_TCPIP_EV_DRIVER,           // Driver event                   driver specific
   MG_TCPIP_EV_USER              // Starting ID for user events
 };