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

CMakeLists.txt

@@ -129,6 +129,10 @@ define_property(GLOBAL PROPERTY PX4_MODULE_PATHS
                  BRIEF_DOCS "PX4 module paths"
                  FULL_DOCS "List of paths to all PX4 modules"
                  )
+define_property(GLOBAL PROPERTY PX4_SRC_FILES
+                 BRIEF_DOCS "src files from all PX4 modules & libs"
+                 FULL_DOCS "SRC files from px4_add_{module,library}"
+                 )
 
 #=============================================================================
 # configuration
@@ -417,6 +421,9 @@ foreach(module ${config_module_list})
 	add_subdirectory(src/${module})
 endforeach()
 
+# add events lib after modules and libs as it needs to know all source files (PX4_SRC_FILES)
+add_subdirectory(src/lib/events EXCLUDE_FROM_ALL)
+
 # must be the last module before firmware
 add_subdirectory(src/lib/parameters EXCLUDE_FROM_ALL)
 target_link_libraries(parameters_interface INTERFACE parameters)