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

src/drivers/drv_led.h

@@ -0,0 +1,61 @@
+/****************************************************************************
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+
+/**
+ * @file drv_led.h
+ *
+ * Led device API to control the external LED(s) via uORB interface
+ */
+
+#pragma once
+
+
+#include <uORB/topics/led_control.h>
+
+#include <board_config.h>
+
+// allow the board to override the number (or maxiumum number) of LED's it has
+#ifndef BOARD_MAX_LEDS
+#define BOARD_MAX_LEDS 4
+#endif
+
+#if BOARD_MAX_LEDS > 8 // because led_mask is uint8_t
+#error "BOARD_MAX_LEDS too large. You need to change the led_mask type in the led_control uorb topic (and where it's used)"
+#endif
+
+
+// set the queue size to the number of LED's, so that each led can be controlled individually
+static const int LED_UORB_QUEUE_LENGTH = BOARD_MAX_LEDS;
+
+
+#define RGBLED0_DEVICE_PATH "/dev/rgbled0"