system: Rename (internal) grub-device to fs->boot-device.

* gnu/system.scm (grub-device): Rename to...
(fs->boot-device): ... this.
(operating-system-grub.cfg): Adapt.
(operating-system-parameters-file): Adapt.

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