templates: Fix bad test on GRUB_DISABLE_SUBMENU

templates: Fix bad test on GRUB_DISABLE_SUBMENU

The GRUB_DISABLE_SUBMENU option is different than the others in the sense
that it has to be set to "y" instead of "true" to be enabled.

That causes a lot of confusion to users, some may wrongly set it to "true"
expecting that will work the same than with most options, and some may set
it to "yes" since for other options the value to set is a word and not a
single character.

This patch changes all the grub.d scripts using the GRUB_DISABLE_SUBMENU
option, so they check if it was set to "true" instead of "y", making it
consistent with all the other options.

But to keep backward compatibility for users that set the option to "y" in
/etc/default/grub file, keep testing for this value. And also do it for
"yes", since it is a common mistake made by users caused by this option
being inconsistent with the others.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>