Support a fonts directory, and make *.pf2 optional (grub, x86)

We check if $dir/*.pf2 exist before trying to copy it. In effect, it means that those fonts are now optional, the script won't fail if there's no such fonts.

We also check if a fonts directory exists, and in such case, we copy it.

Thanks to this change, it becomes possible for debian-installer to put fonts in a grub/fonts/ dir rather than in grub/.

Cf. https://bugs.debian.org/1038862 for details.