tools/atrf-xtal: new utility to measure the crystal frequency

- Makefile (DIRS): added atrf-xtal
- atrf-xtal/Makefile: build atrf-xtal for the Ben or do nothing elsewhere
- atrf-xtal/atrf-xtal.h, atrf-xtal/atrf-xtal.c: diagnostic utility to
  determine the transceiver's crystal frequency
- atrf-xtal/atben.c: dirty low-level driver for ATBEN

tools/atrf-xtal/atrf-xtal.h

@@ -0,0 +1,24 @@
+/*
+ * atrf-xtal/atrf-xtal.h - AT86RF230/1 crystal diagnostic utility
+ *
+ * Written 2011 by Werner Almesberger
+ * Copyright 2011 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#ifndef ATRF_XTAL_H
+#define	ATRF_XTAL_H
+
+#include "atrf.h"
+
+
+void atben_setup(struct atrf_dsc *dsc, int size, int trim);
+unsigned atben_sample(struct atrf_dsc *dsc);
+void atben_cleanup(struct atrf_dsc *dsc);
+
+#endif /* !ATRF_XTAL_H */