Osbuild doesn't support creating btrfs over multiple devices, thus it should
be fine to have only single and dup in the schema (other options are for raid)
So we have a quick reference how to work with btrfs-based images.
Co-authored-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
