system: Initialize console keyboard layout in the initrd.

Partially fixes <https://bugs.gnu.org/25453>.

* gnu/system.scm (<operating-system>)[keyboard-layout]: New field.
(operating-system-initrd-file): Pass #:keyboard-layout to MAKE-INITRD.
* gnu/system/linux-initrd.scm (raw-initrd): Add #:keyboard-layout.
Pass #:keymap-file to 'boot-system'.
(base-initrd): Add #:keyboard-layout.
[helper-packages]: Add LOADKEYS-STATIC when KEYBOARD-LAYOUT is true.
Pass #:keyboard-layout to 'raw-initrd'.
* gnu/build/linux-boot.scm (boot-system): Add #:keymap-file and honor
it.
* doc/guix.texi (operating-system Reference): Document the
'keyboard-layout' field.
(Initial RAM Disk): Update 'raw-initrd' and 'base-initrd' documentation.