gki: Adding GKI_VERSION to fetch corresponding images

Adding GKI_VERSION to kernel source it and
will be maintain in sync with LTS version.
And point to latest GKI images corresponding
to latest LTS tag.

Format of directory holding GKI images will be as
LTS_<LTS version>_<12 character sha key of LTS change>

Example: LTS_5.4.61_8540985fd2ad.

Change-Id: I182e35395e9c870a97982963c749ecf5622cf52a
Signed-off-by: Yogesh Lal <ylal@codeaurora.org>