Fix "Debian GNU/Linux GNU/Linux" menu entries
Maybe not the most elegant solution :/
DPKG_VENDOR and /etc/os-release don't agree on whether or not the GNU/Linux suffix is already present.
/etc/os-release
should always be there when GRUB is being used so let's always use that instead.
Since that already contains the suffix, also stop appending it to the name.
Edited by Mate Kukri