scripts: gki: Add GKI support

Add scripts to enable a build system to compile GKI
based kernels. Specifically, the scripts allows the
build systems to build a particular variant of defconfig
and a menuconfig interface to edit fragments.

Change-Id: I26399ce1911f9484650bb9ee4658c4043c5c556c
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>