Honor GRUB_DISABLE_LINUX_UUID=true for all Debian releases

There's no point in having a release specific check when
checking configuration setting for GRUB_DISABLE_LINUX_UUID.
So if GRUB_DISABLE_LINUX_UUID is set to "true" then always
honor it.

Thanks: hex2a for the initial patch (see PR #96)
Closes grml/grml-debootstrap#87