Display brightness support (#26)

* cleanup

* brightness control and app

* cleanup

* persistant storage of display settings

* fix for missing include

* header cleanup

* fix pc build

* add docs

* move display app to tactility project

boards/yellow_board/display_i.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+bool twodotfour_backlight_init();
+void twodotfour_backlight_set(uint8_t duty);
+
+#ifdef __cplusplus
+}
+#endif