grub: sync with current upstream live ebuild

Minor tweaks, doing this to simplify future changes/maintenance.
grub: add cros-workon based live ebuild, adjust use flags for testing

After this grub unit tests almost work, for 'pc' at least. The only one
in that set that doesn't work are some tests that rely on video
checksums to validate the output. Tests for other targets like 'efi-64'
are a whole new adventure to figure out.