atusb/fw2: moved board-specific functions (SPI, RF reset) out of main()

- Makefile (OBJS): added spi.o
- spi.h, spi.c, atusb.c: moved SPI functions to new file
- spi.h (spi), spi.c (spi), atusb.c (main): renamed function "spi" to
  "spi_io"
- atusb.c (reset_rf, main): moved transceiver reset to separate function

atusb/fw2/spi.h

@@ -0,0 +1,14 @@
+#ifndef SPI_H
+#define	SPI_H
+
+#include <stdint.h>
+
+void spi_begin(void);
+uint8_t spi_io(uint8_t v);
+void spi_end(void);
+void spi_init(void);
+
+#define	spi_send(v)	(void) spi_io(v)
+#define	spi_recv(v)	spi_io(0)
+
+#endif /* !SPI_H */