Comment 2 for bug 1097570

(Note to the enthusiastic: the above diff doesn't actually work so don't waste time building packages of it.)

The way we're calling grub-mkimage causes the prefix to be overridden to (memdisk)/boot/grub, which isn't terribly helpful. Switching the order of -m and -p on the command line improves things here, and then I think it may just be a matter of checking whether $prefix already refers to something that exists on the filesystem before going around searching for things.