Flight termination: lockdown if failure is detected on takeoff

During the first few seconds after takeoff, the failure detector is allowed to
trigger motor lockdown.
This is done for safety reasons to detect tipping-over or unstable
tuning gains

src/modules/commander/Commander.hpp

@@ -309,6 +309,7 @@ private:
 
 	FailureDetector	_failure_detector;
 	bool		_flight_termination_triggered{false};
+	bool		_lockdown_triggered{false};
 
 
 	hrt_abstime	_datalink_last_heartbeat_gcs{0};