atusb/fw: use the unique serial number of the ATmega8/16/32U2 for iSerialNumber

- usb/usb.h (USB_LANGID_ENGLISH_US): added USB LANGID for US-English
- board.h (board_sernum), board.c (board_sernum, hex, get_sernum,
  board_init): provide the board's serial number in "board_sernum"
  (UTF-encoded)
- sernum.h (sernum_get_descr), sernum.c (sernum_get_descr): return
  string descriptors for the serial number
- descr.c (device_descriptor), usb/dfu.c (device_descriptor):
  set iSerialNumber if serial number is available
- atusb.c (main), usb/dfu.c (my_descr): call sernum_get_descr for
  unknown descriptors
- Makefile (OBJS, BOOT_OBJS): added sernum.o

atusb/fw/sernum.h

@@ -0,0 +1,36 @@
+/*
+ * fw/sernum.h - ATUSB serial number
+ *
+ * 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 SERNUM_H
+#define	SERNUM_H
+
+#include <stdint.h>
+
+#include "board.h"
+
+
+#ifdef HAS_BOARD_SERNUM
+
+int sernum_get_descr(uint8_t type, uint8_t index, const uint8_t **reply,
+    uint8_t *size);
+
+#else /* HAS_BOARD_SERNUM */
+
+static inline int sernum_get_descr(uint8_t type, uint8_t index,
+    const uint8_t **reply, uint8_t *size)
+{
+	return 0;
+}
+
+#endif /* !HAS_BOARD_SERNUM */
+
+#endif /* !SERNUM_H */