atusb/fw/: added "lazy enabling" of UART-SPI after ATUSB_GPIO

- spi.h (spi_off), spi.c: disable the UART
- spi.c (spi_initialized, spi_begin, spi_init): initialize the UART
  if necessary
- board_app.c (gpio): call spi_off to disable the UART instead of
  open-coding the operation
- board_app.c (gpio): don't explicitly re-enable the UART but defer it
  to the next communication

atusb/fw/spi.h

@@ -19,6 +19,7 @@
 void spi_begin(void);
 uint8_t spi_io(uint8_t v);
 void spi_end(void);
+void spi_off(void);
 void spi_init(void);
 
 #define	spi_send(v)	(void) spi_io(v)