bug#36137: grub-efi doesn't understand my keyboard-layout

[Julien Lepiller]

Hi!

here is a part of my config:

```
(operating-system
  (keyboard-layout (keyboard-layout "fr" "bepo"))
  (bootloader
    (bootloader-configuration
      (target "/boot/efi")
      (bootloader grub-efi-bootloader)
      (keyboard-layout keyboard-layout)))
  ...)
```

But grub's prompt uses qwerty.