system: grub.cfg uses correct file names when store is not in root partition.

system: grub.cfg uses correct file names when store is not in root partition.

Fixes <http://bugs.gnu.org/24346>.
Reported by csanchezdll@gmail.com (Carlos Sánchez de La Lama).

* guix/scripts/system.scm (previous-grub-entries): Get the initrd file
name from PARAMS.
* gnu/system.scm (operating-system-grub.cfg): Use
'operating-system-initrd-file' to retrieve the initrd file name.
* gnu/system/grub.scm (strip-mount-point): New procedure.
(grub-configuration-file)[entry->gexp]: Call 'strip-mount-point' for
LINUX and INITRD.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>