mavlink & system: add events interface

- sending protocol
- uorb event message & template methods for argument packing
- libevents submodule to send common events and handle json files
- cmake maintains a list of all (PX4) source files for the current build
  (PX4 modules + libs), which is used to extract event metadata and
  generate a json file

msg/CMakeLists.txt

@@ -55,6 +55,7 @@ set(msg_files
 	distance_sensor.msg
 	ekf2_timestamps.msg
 	ekf_gps_drift.msg
+	event.msg
 	esc_report.msg
 	esc_status.msg
 	estimator_event_flags.msg