bug#36876: guix system delete-generations removes custom boot menu entries

[Danny Milosavljevic <dannym@scratchpost.org>]

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Hi Jakob,

keep in mind that switching back to a previous generation could also entail a
switch of bootloader projects.  It does you no good to parse the grub config to
find menu entries and then append them to u-boot.

Fundamentally, it would be best if either the bootloader is part of the state
that guix manages (and each time uses), or it isn't; not both.

Current situation:

* If you select a previous generation in the boot menu, then the state that is
selected does NOT include the bootloader (i.e. it doesn't change the bootloader
or the bootloader config).

* If you do "guix system reconfigure", the state that is saved includes only
the bootloader in the boot sector (simplified) but not the bootloader installer
or the bootloader derivation.

* If you do "guix system delete-generations", the state that is restored does
not include the bootloader installer and previous bootloader configuration.

Clearly, it's not nice to have these different things happen.

It would be better if we retained the generation's bootloader installer
and bootloader config and reinstalled it on each of those.

So, I would suggest to retain the following for each system generation:

* The bootloader package derivation
* The bootloader config derivation
* The bootloader installer derivation

And to install the bootloader each time using the latter.

This introduces cycles--not sure whether that's a problem or not.
For the GC I think it's not a problem.

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