fix prefix and embedded config detection on fedora 18

Fedora patch grub-2.00-for-ppc-include-all-modules-in-the-core-image.patch,
added in RPM grub2-2.00-10, shifts core.img object types numbers,
which prevents BIS from correctly displaying embedded config and prefix.

Unfortunately we have no way to really detect distribution and
RPM version so add some heuristic and detailed explanation for anyone
willing to improve it.

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>
Merge branch 'fu/grub2/fedora'