Add body/NED frame for offboard velocity control

This adds the possibility to use offboard velocity control in the body
frame and not just the NED (world) frame.

The frame is set in the set_position_target_local_ned message and passed
on to mc_pos_control in the position_setpoint topic.

src/modules/mavlink/mavlink_receiver.cpp

@@ -868,6 +868,8 @@ MavlinkReceiver::handle_message_set_position_target_local_ned(mavlink_message_t
 						pos_sp_triplet.current.vy = set_position_target_local_ned.vy;
 						pos_sp_triplet.current.vz = set_position_target_local_ned.vz;
 
+						pos_sp_triplet.current.velocity_frame =
+							set_position_target_local_ned.coordinate_frame;
 					} else {
 						pos_sp_triplet.current.velocity_valid = false;
 					}