General cleanup of /dev/px4io and /dev/px4fmu

- Use distinct common symbols for px4io and px4fmu device files, and use
instead of hardcoded filenames
- Use common symbols defining px4io bits consistently between px4fmu and
px4io builds.

src/drivers/ardrone_interface/ardrone_motor_control.c

@@ -109,7 +109,7 @@ int ar_multiplexing_init()
 {
 	int		fd;
 	
-	fd = open(GPIO_DEVICE_PATH, 0);
+	fd = open(PX4FMU_DEVICE_PATH, 0);
 
 	if (fd < 0) {
 		warn("GPIO: open fail");