Allow installing an "overlay" on the new root partition during upgrade

This is a lightweight mechanism to allow certain changes to persist across upgrades, such as root SSH keys or user home directories

I currently have no need to customize the default image too much, but I do need to configure authorized keys for the root user so that I can log in without a password.

I also thought about adding a hook for installing packages during upgrades to fulfil another common need, but that will be another pull request if I do end up needing it.