FlightTasks: adapt to acceleration based control

src/lib/flight_tasks/tasks/AutoMapper/FlightTaskAutoMapper.cpp

@@ -57,7 +57,7 @@ bool FlightTaskAutoMapper::update()
 	// vehicle exits idle.
 
 	if (_type_previous == WaypointType::idle) {
-		_thrust_setpoint.setNaN();
+		_acceleration_setpoint.setNaN();
 	}
 
 	// during mission and reposition, raise the landing gears but only
@@ -122,7 +122,7 @@ void FlightTaskAutoMapper::_prepareIdleSetpoints()
 	// Send zero thrust setpoint
 	_position_setpoint.setNaN(); // Don't require any position/velocity setpoints
 	_velocity_setpoint.setNaN();
-	_thrust_setpoint.zero();
+	_acceleration_setpoint = Vector3f(0.f, 0.f, 100.f); // High downwards acceleration to make sure there's no thrust
 }
 
 void FlightTaskAutoMapper::_prepareLandSetpoints()