RC_Channel: added support for LimitValue settings

this allows you to set a channel failsafe or radio_out to a limit
value
APM_OBC: setup termination values in PX4IO

this sets up the PX4IO board with failsafe values in case the FMU is
not running
Plane: use new APM_OBC API
Plane: setup failsafe values for OBC in failsafe callback