Plane: rename old channel_* globals

prevent the old "g." names from being used
Plane: make it possible to change control channel ordering without reboot

this makes configuration a bit easier
AP_AHRS: make it possible to setup board orientation at runtime
Plane: make it possible to change board orientation at runtime
Copter: make it possible to change board orientation at runtime
Rover: make it possible to change board orientation at runtime
RC_Channel: removed reference to dip switches