gnu: grub: Use installed unicode font.

* gnu/bootloader/grub.scm (setup-gfxterm): Remove parameter font-file.
Refer to the installed font in /boot, not in the store.
(font-file): Delete obsolete function.
(install-grub): When populating a disk-image, install the unicode font, too.

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