Code review comment for ~juliank/grub/+git/ubuntu:resilient-boot

I'm happy now, please review.

Use cases:

* New install:
* Upgrade:

If you have not preseeded grub2/efi_install_devices, we'll transition the device that is currently mounted to /boot/efi into the setting, so that you don't get asked. Doing this on install also keeps existing installers working, they can then add the debconf magic to get resilient boot support.

* User changed /boot/efi to point elsewhere not yet configured as an ESP

We take the current debconf settings, append the device that /boot/efi points to to the list of enabled things, and then prompt the user if that's correct.

* User unmounted /boot/efi

If the user unmounted /boot/efi during upgrade/install, they'll get prompted for which ESPs to install. If the user unmounted /boot/efi otherwise, the device that was mounted there will be mounted to /var/lib/grub/esp as for any other ESP.

* User does not have an ESP-type partition, or the wanted one does not have an ESP identifier

This might happen if the system supports booting from any FAT partition, not just those with magic ESP identifiers. Our installer does not create such partitions, though. The user will see an empty list, or well, at least not the partition they want.

* User mounted sth to /boot/efi that is not an ESP

Similar to before, except the user will get prompted on each upgrade.