Add a small LED interface and demo app.

app/dartino/leds.c

Index: app/dartino/leds.c
diff --git a/app/dartino/leds.c b/app/dartino/leds.c
new file mode 100644
index 0000000000000000000000000000000000000000..ceeebc31578979a9eae4ebf8434fd64c56aa7c61
--- /dev/null
+++ b/app/dartino/leds.c
@@ -0,0 +1,30 @@
+
+// Copyright (c) 2015, the Dartino project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE.md file.
+
+#include <kernel/port.h>
+
+#if defined(TARGET_DARTUINOP0)
+#include <dev/gpio.h>
+#include <target/gpioconfig.h>
+
+int led_get_gpio(int led) {
+  switch (led) {
+    case 0:
+      return GPIO_LED108;
+    case 1:
+      return GPIO_LED109;
+    case 2:
+      return GPIO_LED110;
+    case 3:
+      return GPIO_LED111;
+    default:
+      return -1;
+  }
+}
+#else
+int led_get_gpio(int led) {
+  return -1;
+}
+#endif