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.c

@@ -19,8 +19,13 @@
 #include "spi.h"
 
 
+static int spi_initialized = 0;
+
+
 void spi_begin(void)
 {
+	if (!spi_initialized)
+		spi_init();
 	CLR(nSS);
 }
 
@@ -41,6 +46,13 @@ void spi_end(void)
 }
 
 
+void spi_off(void)
+{
+	spi_initialized = 0;
+	UCSR1B = 0;
+}
+
+
 void spi_init(void)
 {
 	SET(nSS);
@@ -55,4 +67,6 @@ void spi_init(void)
 	UCSR1B = 1 << RXEN1 | 1 << TXEN1;
 			/* enable receiver and transmitter */
 	UBRR1 = 0;	/* reconfirm the bit rate */
+
+	spi_initialized = 1;
 }