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/event.msg

@@ -0,0 +1,11 @@
+# Events interface
+uint64 timestamp			# time since system start (microseconds)
+
+uint32 id                   # Event ID
+uint16 sequence             # Sequence number
+uint8[25] arguments         # (optional) arguments, depend on event id
+
+uint8 log_levels            # Log levels: 4 bits MSB: internal, 4 bits LSB: external
+
+uint8 ORB_QUEUE_LENGTH = 8
+