Key increment scripts: fix incrementing keys not in $PWD

Part of the recent rewrite dropped changing of the active dir to
${KEY_DIR}.  Meant the scripts needed to be run inside of the key
dir since not all functions take the key dir as an argument but
instead assume they're in ${PWD}.

BUG=None
TEST=Ran increment_kernel_subkey_and_key_mp.sh and saw it work
BRANCH=None

Change-Id: Icbc02f123e999d186d9c40fd16528a134397699e
Reviewed-on: https://gerrit.chromium.org/gerrit/35803
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
5 files changed