kirkwood: fix sysupgrade

The platform_check_image() stub need to return 0 for success, otherwise
the sysupgrade will fail with:

  Image check 'platform_check_image' failed.

Fixes: aa6f5f1787 ("kirkwood: use image metadata")
Signed-off-by: 尤晓杰 <yxj790222@163.com>
[reworded commit message}
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/kirkwood/base-files/lib/upgrade/platform.sh

@@ -4,7 +4,7 @@ RAMFS_COPY_DATA='/etc/fw_env.config /var/lock/fw_printenv.lock'
 REQUIRE_IMAGE_METADATA=1
 
 platform_check_image() {
-	return 1
+	return 0
 }
 
 platform_do_upgrade() {