Support sending SET_GPS_GLOBAL_ORIGIN message for Copter and Rover (at least)

[rmackay9]

In Copter-3.5.4 and Rover-3.2-rc3 (and higher) if the user wishes to use non-GPS navigation (i.e. the Pozyx system, visual odometry, etc) then the EKF must be told where the EKF origin should be placed using the SET_GPS_GLOBAL_ORIGIN message (http://mavlink.org/messages/common#SET_GPS_GLOBAL_ORIGIN). We should add a new option to the right-mouse-button menu called "Set EKF Origin Here". This will be a very rarely used message but I don't think we should hide it on the Ctrl-F menu.

By the way, in older versions of the vehicle firmware we used the SET_HOME_POSITION message (http://mavlink.org/messages/common#SET_HOME_POSITION) to set the EKF origin but this led to some issues with callers accidentally setting the EKF origin when they really only wanted to set home.

We need this message for both Copter and Rover.

[meee1]

I assume this message is guarded on copter/rover side to prevent crashing?

[meee1]

currently MP implemented no safeguards around this. I could check for if armed?

[rmackay9]

Thanks!

@meee1, I don't think it should be necessary to check anything on the MP side. In ardupilot we check the message to make sure it's a valid lat/lon/alt. Also we only accept setting the origin once and most users will want to set it before arming.