* 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>