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/systemcmds/tests/test_gpio.c

@@ -92,7 +92,7 @@ int test_gpio(int argc, char *argv[])
 	int		fd;
 	int		ret = 0;
 
-	fd = open(GPIO_DEVICE_PATH, 0);
+	fd = open(PX4IO_DEVICE_PATH, 0);
 
 	if (fd < 0) {
 		printf("GPIO: open fail\n");