With the ro image we have a cheap way to figure out which
files were changed. We can run cmp between the to be backed
up file. This way the stored data will be minimal in the
sense of changed files.
When using squashfs + overlayfs a root, we need to store the backup
somewhere else and restore it on every boot. Do avoid breaking existing
users, this is implemented in a separate recipe (as suggested by Holger).