Offboard only RC lost failsafe

[benjinne]

Describe problem solved by the proposed feature
For my use case, I need a way to failsafe only on RC lost when using offboard. This is because we sometimes have issues with our RC transmitter when flying offboard and we need to be able to take over at any time otherwise the RC lost failsafe must execute.

Describe your preferred solution
I've got 3 proposals to address my solution in order that I desire.

1. Add a rc_lost condition to the code here:

   PX4-Autopilot/src/modules/commander/state_machine_helper.cpp

   Line 684 in 04f9ada

   case commander_state_s::MAIN_STATE_OFFBOARD:

    if (status_flags.offboard_control_signal_lost) {
    	if (status->rc_signal_lost) {
    		// Offboard and RC are lost
    		enable_failsafe(status, old_failsafe, mavlink_log_pub, reason_no_rc_and_no_offboard);
    		set_offboard_loss_nav_state(status, armed, status_flags, offb_loss_act);
   
    	} else {
    		// Offboard is lost, RC is ok
    		enable_failsafe(status, old_failsafe, mavlink_log_pub, reason_no_offboard);
    		set_offboard_loss_rc_nav_state(status, armed, status_flags, offb_loss_rc_act);
    	}
   
    } else if (rc_lost) {
    	/* RC is lost, when required for safety */
    	enable_failsafe(status, old_failsafe, mavlink_log_pub, reason_no_rc);
    	set_link_loss_nav_state(status, armed, status_flags, internal_state, rc_loss_act);
   
    } else {
    	status->nav_state = vehicle_status_s::NAVIGATION_STATE_OFFBOARD;
    }
   

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2. Add another parameter to failsafe in offboard when only RC is lost.
3. Use the offboard controller to sense RC is lost then change the mode to failsafe.

This first solution changes the existing standard that when RC is lost it should not affect any automatic modes. To keep this functionality, the rc failsafe can be set to disabled so that way it will not affect offboard, however this would remove the following use case: when the user wants rc failsafe enabled in manual modes but not in offboard for the same flight.