lib/led: add led class with led_control uorb topic

This is the new interface to control external LED's (user-facing).
Features:
- Supports maximum N Leds (where the board can define N)
- on/off/blink M times
- Different priorities
- Allows setting a single led or multiple at once

msg/CMakeLists.txt

@@ -67,6 +67,7 @@ set(msg_file_names
 	hil_sensor.msg
 	home_position.msg
 	input_rc.msg
+	led_control.msg
 	log_message.msg
 	manual_control_setpoint.msg
 	mavlink_log.msg