Plane: changed preflight calibration to prevent accel cal from MP

MissionPlanner sends param1=1 and param3=1 for preflight
calibration. This was having the effect of redoing the accel
calibration as 1D cal on every flight!
Rover: make preflight calibration follow plane conventions
Plane: prepare for 3.1.2beta3
Plane: if SKIP_GYRO_CAL is set the do a gyro cal on 3D accel cal

otherwise the user has no opportunity to do a gyro calibration
Copter: only allow one preflight calibration at a time