atusb/fw2/: move board-specific items to board-specific file

- io.h: renamed to board.h
- Makefile (OBJS): added board.o
- atusb.c (reset_rf): moved to board.c
- board.h (reset_rf): give reset_rf a proper declaration
- atusb.c, ep.c, spi.c: include board.h instead of io.h

atusb/fw2/ep0.c

@@ -27,7 +27,7 @@
 #include "at86rf230.h"
 #include "atusb/ep0.h"
 //#include "version.h"
-#include "io.h"
+#include "board.h"
 #include "spi.h"
 
 
@@ -35,9 +35,6 @@
 static const char *build_date = "today";
 static unsigned build_number = 42;
 
-extern void reset_rf(void);
-
-
 #define debug(...)
 #define error(...)