atusb/fw2: further abstract board functions

- atusb.c (main), board.h (board_init), board.c (board_init): moved
  clock and I/O initialization to board file
- board.h (led), board.c (led), atusb.c (main): abstracted LED setting
  in function "led" instead of open-coding it
- spi.c (spi_init): set nSS to 1 before enabling the output, so that we
  don't generate what looks like an empty SPI transaction

atusb/fw2/spi.c

@@ -30,6 +30,7 @@ void spi_end(void)
 
 void spi_init(void)
 {
+	SET(nSS);
 	OUT(SCLK);
 	OUT(MOSI);
 	OUT(nSS);