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/board.c

@@ -15,15 +15,20 @@
 
 #include <avr/io.h>
 #include <avr/interrupt.h>
+#include <avr/boot.h>
 
 #define F_CPU   8000000UL
 #include <util/delay.h>
 
+#include "usb.h"
 #include "at86rf230.h"
 #include "board.h"
 #include "spi.h"
 
 
+uint8_t board_sernum[42] = { 42, USB_DT_STRING };
+
+
 static void set_clkm(void)
 {
 	/* switch CLKM to 8 MHz */
@@ -94,6 +99,25 @@ void panic(void)
 }
 
 
+static char hex(uint8_t nibble)
+{
+	return nibble < 10 ? '0'+nibble : 'a'+nibble-10;
+}
+
+
+static void get_sernum(void)
+{
+	uint8_t sig;
+	int i;
+
+	for (i = 0; i != 10; i++) {
+		sig = boot_signature_byte_get(i+0xe);
+		board_sernum[(i << 2)+2] = hex(sig >> 4);
+		board_sernum[(i << 2)+4] = hex(sig & 0xf);
+	}
+}
+
+
 void board_init(void)
 {
 	/* Disable the watchdog timer */
@@ -111,6 +135,8 @@ void board_init(void)
 	/* set up all the outputs; default port value is 0 */
 
 	OUT(LED);
-	OUT(nRST_RF);   /* resets the transceiver */
+	OUT(nRST_RF);   /* this also resets the transceiver */
 	OUT(SLP_TR);
+
+	get_sernum();
 }