added GlobalTime message

allowing peripherals access to global time stamp without direct GPS
access

Author: tridge

dronecan/protocol/344.GlobalTime.uavcan

@@ -0,0 +1,5 @@
+#
+# provide global timestamp, time since 1/1/1970 in microseconds
+# should only be broadcast once a reliable time base (GPS or equivalent) is known
+#
+uavcan.Timestamp timestamp