Writeable /etc

[antonlacon]

This makes /etc/ writeable via an overlayfs backed by /storage. I'm using it in testing other things (users/groups), but given the number of symbolic links coming out of /etc to /storage or /var, I'm sharing it.

Labeled WIP because although it's functionally complete, storage-dirs.service should probably be replaced by tempfiles.d usage. Maybe move the systemd files from busybox to systemd's package too. Busybox's package was picked because it has the var.mount file too.

Blocked for discussion.

[antonlacon]

Replaced storage-dirs.service with a tmpfiles.d method. I'm not aware of how the numbering/naming of the tmpfiles.d files works, so I picked early, but not first.