Respect GRUB_DISABLE_LINUX_PARTUUID setting

[fingolfin0]

Note that grub-mkconfig uses PARTUUID even if the setting GRUB_DISABLE_LINUX_UUUID=true is not specified. It is sufficient only to specify the setting GRUB_DISABLE_LINUX_PARTUUID=false. At least that's how it works on my system (gentoo). Here is an excerpt from the grub-mkconfig documentation:

If ‘grub-mkconfig’ cannot identify the root filesystem via its
universally-unique indentifier (UUID), ‘grub-mkconfig’ can use the
UUID of the partition containing the filesystem to identify the
root filesystem to the Linux kernel via a ‘root=PARTUUID=...’
kernel parameter. This is not as reliable as using the filesystem
UUID, but is more reliable than using the Linux device names. When
‘GRUB_DISABLE_LINUX_PARTUUID’ is set to ‘false’, the Linux kernel
version must be 2.6.37 (3.10 for systems using the MSDOS partition
scheme) or newer. This option defaults to ‘true’. To enable the
use of partition UUIDs, set this option to ‘false’.