add multicast support

[shiv-tyagi]

This adds multicast support to the mavesp8266 firmware. The user can select between unicast and multicast mode using the web interface and specify the multicast ip and the port. The multicast functionality works only when the esp module is connected to an access point using the STA interface. When doing multicast, the esp modules can communicate with each other on the specified multicast ip and port. The default multicast ip and port are 239.255.145.50 and 14550 respectively. The user can connect mavproxy to all the vehicles in multicast mode using the command mavproxy.py --master mcast: --console --map (if the multicast ip and port are set to default and the gcs is also connected to the same access point).