Add variable 'grubdisk' to export boot disk

Boot disk name is required by 'gptpriority' grub command to read the
priority of partitions from the GPT disk and set the default
partition to boot accordingly in grub.cfg

BUG=None
TEST=Built the changes along with chromeOS. Verified the value being
     exported to grubdisk in grub shell and also the usage of this
     variable with "gptprioriry" grub command in grub.cfg.

Change-Id: Ib52de5e2ab32eb9538b5f537d11c549eeb25278d
Reviewed-on: https://gerrit.chromium.org/gerrit/58273
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Josh Triplett <josh@joshtriplett.org>
Tested-by: Josh Triplett <josh@joshtriplett.org>
1 file changed