Factory reset of the overlayed files

[deribaucourt]

Is your feature request related to a problem? Please describe.
Anytime a file which as a /mnt/persistent overlay is edited, it will be saved there. The issue is that it won't be updated with a swupdate.

Describe the solution you'd like
Most distros using peristent overlays allow resetting the overlays (basically removing all the upper dirs). This can be implemented as a token file to remove in /mnt/persistent/remove_me_to_reset. An early systemd unit before mounting the overlay would wipe them.

Describe alternatives you've considered
Manually resetting the files to their original contents.

Additional context