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.

platforms/common/uORB/uORB.cpp

@@ -51,6 +51,7 @@
 #include <sys/boardctl.h>
 #endif
 
+
 static uORB::DeviceMaster *g_dev = nullptr;
 
 int uorb_start(void)
@@ -113,6 +114,18 @@ int uorb_top(char **topic_filter, int num_filters)
 	return OK;
 }
 
+void uorb_shutdown(void)
+{
+#ifdef CONFIG_ORB_COMMUNICATOR
+	uORBCommunicator::IChannel *ch = uORB::Manager::get_instance()->get_uorb_communicator();
+
+	if (ch) {
+		ch->shutdown();
+	}
+
+#endif /* CONFIG_ORB_COMMUNICATOR */
+}
+
 orb_advert_t orb_advertise(const struct orb_metadata *meta, const void *data)
 {
 	return uORB::Manager::get_instance()->orb_advertise(meta, data);