system: grub: Allow arbitrary kernel file names in 'menu-entry'.

Fixes <http://bugs.gnu.org/20067>.
Reported by Tomáš Čech <sleep_walker@suse.cz>.

* gnu/system.scm (system-linux-image-file-name)
(operating-system-kernel-file): New procedures.
(operating-system-grub.cfg): Use 'operating-system-kernel-file' for the
'kernel' field of 'menu-entry'.
(operating-system-parameters-file): Likewise for the 'kernel' entry.
(read-boot-parameters): Adjust 'kernel' field so that it contains the
absolute file name of the image.
* gnu/system/grub.scm (grub-configuration-file)[linux-image-name]:
Remove.
[entry->gexp]: Assume LINUX is the absolute file name of the kernel
image.
* doc/guix.texi (GRUB Configuration): Add an example, and adjust
'kernel' field documentation accordingly.