scripts: gki: Clean the generted files by hand instead of mrproper

The script generate_defconfig.sh runs 'make mrproper' at the end
as a part of its cleanup routine. This action, however, also removes
tags, cscope.*, and other necessary files from the source tree,
which is not desired. Hence, hand-pick the generated files manually
and remove them instead of relying on 'make mrproper'.

Change-Id: I8996cb7be5574d6177b23b72bf3e241d43a350dd
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>