ability to forward GLOBAL_POSITION_INT between links

[tridge]

For use in ship landing we need to get GLOBAL_POSITION_INT between a ground beacon and the aircraft. While we recommend a mesh radio setup, it would be good to be able to do this in a simpler fashion with the ground beacon being connected on USB to the GCS. In MAVProxy you can do "set fwdpos 1" and the two packets will be forwarded between the connected aircraft. It would be nice to have a tickbox for this in MissionPlanner to allow for simplified ship landing setups.

mavproxy implementation is here: https://github.com/ArduPilot/MAVProxy/blob/master/MAVProxy/modules/mavproxy_link.py#L819