Tested with:
- boot from kernel + initrd with metadata (appears as sr0)
- boot from iso with no metadata (root is sr0)
- boot from iso with metadata (root is sr0, metadata is sr1)
- boot from iso with metadata using containerized qemu
In all cases where it was present the metadata was correctly expanded to
/var/config.
Signed-off-by: Ian Campbell <ijc@docker.com>