Allow custom BTRFS subvolume layouts

[Winterhuman]

Being able to customise the BTRFS layout would be a nice addition, especially for applications like snapper which like flat subvolume layouts such as:

... level 5 path @
... level 5 path @home
... level 5 path @snapshots
...

This does complicate things slightly since I think mkosi relies on root being the top-level subvolume so that rootflags=subvol=@ isn't necessary in fstab or dracut.conf.

[behrmann]

I'd happily review a PR. This would definitely be interesting together with systemd/systemd#21353.

[DaanDeMeyer]

This is something that should be supported in systemd-repart, which itself requires support for creating subvolumes from mkfs.btrfs. Anyway, since it should be implemented in systemd-repart, let's close the issue here