Add PS/2 mouse support to connect TrackPoint Unit.

Change build options:  Makefile and config.h. See README.

mousekey.h

@@ -0,0 +1,11 @@
+#ifndef MOUSEKEY_H
+#define  MOUSEKEY_H
+
+#include <stdbool.h>
+
+void mousekey_decode(uint8_t code);
+bool mousekey_changed(void);
+void mousekey_usb_send(void);
+
+#endif
+