FlightTasks: add Descend task to land without GPS

This adds a flight task to catch the case where we want to do an
emergency descent without GPS but only a baro.

Previously, this would lead to the navigator land class being called
without position estimates which then made the flight tasks fail and
react with a flight task failsafe. This however meant that landed was
never detected and a couple of confusing error messages.

This applies if NAV_RCL_ACT is set to 3 "land".

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -845,6 +845,28 @@ MulticopterPositionControl::start_flight_task()
 			// we want to be in this mode, reset the failure count
 			_task_failure_count = 0;
 		}
+
+	} else if (_vehicle_status.nav_state == vehicle_status_s::NAVIGATION_STATE_DESCEND) {
+
+		// Emergency descend
+		should_disable_task = false;
+		FlightTaskError error = FlightTaskError::NoError;
+
+		error =  _flight_tasks.switchTask(FlightTaskIndex::Descend);
+
+		if (error != FlightTaskError::NoError) {
+			if (prev_failure_count == 0) {
+				PX4_WARN("Descend activation failed with error: %s", _flight_tasks.errorToString(error));
+			}
+
+			task_failure = true;
+			_task_failure_count++;
+
+		} else {
+			// we want to be in this mode, reset the failure count
+			_task_failure_count = 0;
+		}
+
 	}
 
 	// manual position control