Add support for multiple BTRFS devices
With storage plugin and rollbacks it is problematic that all files on external storage are kept as they were. This can cause rollback to break anything that has files in
At the same time it is beneficial to have other devices that are no way the same way tied to running root (data only) to use schnapps as well.
In the end this leads to need have support for multiple devices in schnapps.
The idea would be that additional mount points (or devices) could be configured in schnapps in two possible modes. Either tied to root filesystem or not tied. With tied devices all operations done as regular with schnapps would affect not only root filesystem but also all tied ones.
The only problem there is rescue. It is probably bad idea to implement schnapps in rescue in fullest. That means reading configuration in target system and rolling back other devices as well (we might not have access on them from rescue kernel for example as one of the possible problems). This could be solved by script executed at boot that would check current running snapshot, read configuration and modified other tied filesystems accordingly (probably before they are mounted for the first time using blockd or storage plugin).