Grub menu not displayed

[Tanguy LE CARROUR <tanguy@bioneland.org>]

Hi Guix,

I've been having this problem since the very beginning but, as it is not
a "must have" and it only affects 1 of my 3 computers running Guix System,
I've lived without the Grub menu for years.

When my computer boots, I only see a black screen with "Seabios […] Grub
loading […] Welcome to Grub!", but I never get the Grub menu where I can select
a previous generation. After a few seconds, it boots and I get to the
graphical session manager where I can login.

My configuration is the following:

```
  (bootloader (bootloader-configuration
                (bootloader grub-bootloader)
                (targets (list "/dev/sda"))
                (keyboard-layout keyboard-layout)))
```

My guess is that it's related to the screen resolution. So I had a look
into `gnu/bootloader/grub.scm` and saw the following:

```
 (define-record-type* <grub-theme>
 ;; Default theme contributed by Felipe López.
  grub-theme make-grub-theme
  grub-theme?
   (image           grub-theme-image
                    (default (file-append %artwork-repository
                                          "/grub/GuixSD-fully-black-4-3.svg")))
   (resolution      grub-theme-resolution
                    (default '(1024 . 768)))
   (color-normal    grub-theme-color-normal
                    (default '((fg . light-gray) (bg . black))))
   (color-highlight grub-theme-color-highlight
                    (default '((fg . yellow) (bg . black))))
   (gfxmode         grub-theme-gfxmode
                    (default '("auto"))))          ;list of string
```

Before I try to define a custom `grub-theme` record, to change the `resolution`
or the `gfxmode`, I wanted to ask you guys, for if I mess up my Grub,
I might be in big trouble! ^_^'

Any help welcome!

Regards,

-- 
Tanguy