* util/update-grub_lib.in (convert_system_path_to_grub_path): Split
code to make path relative to its root into a separate function.
* util/i386/pc/grub-install.in: Use newly provided
make_system_path_relative_to_its_root() to convert ${grubdir}, then
pass the result to grub-install --prefix.
that I forgot to apply before.