common.xml: add new frame MAV_FRAME_GLOBAL_VEHICLERELATIVE_ALT

[peterbarker]

this is expected to be used to tell a vehicle to take off to 20m above current altitude.

ArduPlane already actually does this for some commands, but uses an inappropriate altitude frame.

Should also be useful for "go up 20m".

[rmackay9]

TBH, I think this just adds more complexity than it's worth. Let's make the vehicle accept a takeoff command to a specific alt-vs-home, alt-vs-terrain or alt-vs-sea-level instead of adding a new frame to match what the code does now.

I just worry that if we add this frame here then we will need to add it to every mission command we support.