Comment 1 for bug 2054103

First we are going to change grub to not hard-error when `grub-pc/install_devices` is empty, in order to avoid worse regressions than grub-pc simply being outdated on disk.

Second, since we know the image layout on cloud images, we can use `grub-probe` to determine the install device based on the filesystem containing the `/boot` directory.

We are introducing the `grub-{efi,pc}/cloud_style_installation` debconf option to determine the install device using `grub-probe` dynamically instead of having to fill the `grub-pc/install-devices`debconf entry.

This can then be set in cloud images at build time, and reinstallation should just work this way without having to introduce a new service.