logging: publish a message for PX4_{WARN,ERR} & store them to the ulog file

- ulog file message rate limited to 50Hz
- queuing with size 2
- this replaces the mavlink log message in the ulog
  (but the mavlink warnings & errors still go to the ulog)

msg/CMakeLists.txt

@@ -65,6 +65,7 @@ set(msg_file_names
 	hil_sensor.msg
 	home_position.msg
 	input_rc.msg
+	log_message.msg
 	manual_control_setpoint.msg
 	mavlink_log.msg
 	mc_att_ctrl_status.msg

msg/log_message.msg

@@ -0,0 +1,5 @@
+# A logging message, output with PX4_{WARN,ERR,INFO}
+
+uint8 severity # log level (same as in the linux kernel, starting with 0)
+uint8[127] text
+