Notify: add pre_arm_gps_check flag

RGB LED will remain flashing blue when vehicle is disarmed and this
check has failed (i.e. false).
Copter: set pre_arm_gps_check flag
Plane: pre_arm_gps_check flag always true
Rover: pre_arm_gps_check flag always true
Tracker: pre_arm_gps_check flag always true