extmod/modlwip: Call user callback on newly-received UDP or RAW packet.

User callbacks allow code to respond to incoming messages without blocking
or polling.  User callbacks are optional, and if no callback is registered
the code has no effect.

The mechanism is the same as for TCP: when a connection is accepted or a
TCP packet is received, a user callback is executed.

Fixes issue micropython#3594.

Signed-off-by: Jack Whitham <jack.d.whitham@gmail.com>

Author: jwhitham

extmod/modlwip.c

@@ -463,6 +463,8 @@ static void udp_raw_incoming(lwip_socket_obj_t *socket, struct pbuf *p, const ip
         slot->peer_addr = *addr;
         slot->peer_port = port;
         socket->incoming.udp_raw.iput = (socket->incoming.udp_raw.iput + 1) % LWIP_INCOMING_PACKET_QUEUE_LEN;
+        // Notify user callback of the new packet
+        exec_user_callback(socket);
     }
 }