system: Pass boot-parameters to (gnu system grub).

* gnu/system.scm (operating-system-bootcfg): Pass boot-parameters.
* gnu/system/grub.scm (boot-parameters->menu-entry): New variable.
(grub-configuration-file): Use boot-parameters->menu-entry.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>