galileo: Add board-level abstraction layer for GPIO

This patch adds a HAL for GPIOs so that users of the API can specify
board-level IO pin numbers rather than CPU-level pin numbers.

examples/galileo/gpio-output.c

@@ -33,9 +33,10 @@
 #include "contiki.h"
 #include "sys/ctimer.h"
 
+#include "galileo-gpio.h"
 #include "gpio.h"
 
-#define PIN 5 /* IO2 */
+#define PIN 2
 
 static uint32_t value;
 static struct ctimer timer;
@@ -48,7 +49,7 @@ timeout(void *data)
 {
   /* toggle pin state */
   value = !value;
-  quarkX1000_gpio_write(PIN, value);
+  galileo_gpio_write(PIN, value);
 
   ctimer_reset(&timer);
 }