uORB: Added a new uorb_shutdown function that is called during normal shutdown procedures. It will only

call into a new UORB COMMUNICATOR ICHANNEL shutdown interface if it has been configured, otherwise it
does nothing. This allows ICHANNEL implementations to pass on a shutdown indication to a remote processor.
Implemented the shutdown interface in the muorb module for VOXL flight controllers.

src/modules/muorb/apps/muorb_main.cpp

@@ -58,7 +58,9 @@ muorb_init()
 	if (channel && channel->Initialize(enable_debug)) {
 		uORB::Manager::get_instance()->set_uorb_communicator(channel);
 
-		if (channel->Test()) { return OK; }
+		if (channel->Test()) {
+			return OK;
+		}
 	}
 
 	return -EINVAL;