Add hooks for deployment steps to allow bootloader swapping

[Mstrodl]

Can be used to provide hooks into a sysroot deploy for tools like bootupd to perform bootloader swaps.

Happy to do this a different way, but this is how we've been doing it on our debian-based system. I understand bootupd is supposed to work a different way. We do it this way because grub-mkconfig will generate a broken config for the older version of grub.

See #3496 for motivation

[openshift-ci[bot]]

Hi @Mstrodl. Thanks for your PR.

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[Mstrodl]

By the way if this is not something you guys want to take, that's fine too. I just figured other people might find it useful too :)

[champtar]

One important info from #3496 that you forgot to repeat here is that Debian version of grub doesn't support BLS (yet), so you can't use bootupd static config :(