How to use custom grub?

[Marius Bakke <mbakke@fastmail.com>]

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Greetings Guix,

I have a working UEFI GuixSD system, by adding FAT32 support to
base-initrd and installing the initial UEFI bootloader from a non-Guix
live CD.

Now that it's booted, I want to use a custom 'grub-efi' package so that
`guix system reconfigure` works without passing --no-grub and manually
copying grub.cfg.

The "grub-configuration" object takes a 'grub' argument which defaults
to (@ (gnu packages grub) grub). However setting that to 'grub-efi' in
the operating-system declaration or even directly in gnu/system/grub.scm
has no effect.

Looking at guix/scripts/system.scm, grub is defined as
(package->derivation grub). I tried exporting grub-configuration-grub
and using that in the (package->derivation) procedure, but that broke
the world :)

What is the correct approach here?

Additionally the "grub-install" arguments are slightly different for EFI
systems. Tips for properly handling that appreciated.

Thanks!

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