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

src/systemcmds/topic_listener/CMakeLists.txt

@@ -53,7 +53,7 @@ px4_add_module(
 		${CMAKE_CURRENT_BINARY_DIR}
 	SRCS
 		listener_main.cpp
-		listener_generated.cpp
+		${CMAKE_CURRENT_BINARY_DIR}/listener_generated.cpp
 	DEPENDS
 		generate_topic_listener
 	)