feature: support updating the recovery img from the live rootfs (to allow resetting to some previous state)

[tolland]

Ticket mentions a use case where they want to restore to some non-factory state:
#13

This should be plausibly possible. Main issue would seem to be the auto-resizer that runs on first boot, as once the image has been resized to fill the remaining space, it would possibly be too big to use it anymore.

• possibly re-shrink the image? (following the auto-resizer)
• have an option to boot from recovery partition, but not auto-restore
• have an intermediate first boot, which allows the user to configure the rPi, and disable the auto-resizer until second boot, or when required.

[ahsu-2019]

bump