Add support for .xbm bitmap file

.xbm is easy to include in C as source file. It could be viewed
and edited directly without need to be converted between images
and C array.

src/resource.h

@@ -0,0 +1,10 @@
+#ifndef __RES_H__
+#define __RES_H__
+
+#include <stdint.h>
+#include "xbm.h"
+
+extern xbm_t bluetooth;
+extern xbm_t splash;
+
+#endif /* __RES_H__ */