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/usb/usb.h

@@ -1,8 +1,8 @@
 /*
  * fw/usb//usb.h - USB hardware setup and standard device requests
  *
- * Written 2008, 2009 by Werner Almesberger
- * Copyright 2008, 2009 Werner Almesberger
+ * Written 2008, 2009, 2011 by Werner Almesberger
+ * Copyright 2008, 2009, 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
@@ -74,6 +74,14 @@
 #define	SET_INTERFACE		0x0b
 #define	SYNCH_FRAME		0x0c
 
+/*
+ * USB Language ID codes
+ *
+ * http://www.usb.org/developers/docs/USB_LANGIDs.pdf
+ */
+
+#define	USB_LANGID_ENGLISH_US	0x409
+
 
 /*
  * Odd. sdcc seems to think "x" assumes the size of the destination, i.e.,