* util/update-grub_lib.in (font_path): New function. Determine wether
a font file can be found and, if so, echo the GRUB path to it.
* util/update-grub.in: Handle multiple terminals depending on user
input, platform availability and font file presence. Propagate
variables of our findings to /etc/grub.d/ children.
* util/grub.d/00_header.in: Handle multiple terminals, based on
environment setup by update-grub.
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