add GRUB_DISABLE_SUBMENU option

This patch adds the ability to disable the grub2 submenus from
/etc/default/grub

To disable the submenus

echo 'GRUB_DISABLE_SUBMENU="true"' >> /etc/default/grub
blscfg: add blscfg module to parse Boot Loader Specification snippets

http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec

Works like this:

 insmod blscfg
 bls_import

Done! You should now have menu items for your snippets in place.

Signed-off-by: Peter Jones <grub2-owner@fedoraproject.org>
Fix docs

I think the doc generation tools got stricter, so fix up the .texi file
to work.