remoteid: implement System as sent from GCS

This will send the System message if it is already being sent by a ground
station. Otherwise, it will assemble the message itself using the
takeoff/home location.

msg/CMakeLists.txt

@@ -155,6 +155,7 @@ set(msg_files
 	OpenDroneIdArmStatus.msg
 	OpenDroneIdOperatorId.msg
 	OpenDroneIdSelfId.msg
+	OpenDroneIdSystem.msg
 	OrbitStatus.msg
 	OrbTest.msg
 	OrbTestLarge.msg