* gnu/bootloader/grub.scm (keyboard-layout-file): New procedure.
(grub-configuration-file)[keyboard-layout-file]: New variable.
[builder]: Use it.
* gnu/bootloader.scm (<bootloader-configuration>)[keyboard-layout]: New
field.
* doc/guix.texi (Bootloader Configuration): Document it.
Co-authored-by: nee <nee-git@hidamari.blue>