certify_bootimg: makes gki-info.txt optional am: 79627361ad

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2051402

Change-Id: Ife315c05835675c8f6944b18b1d3685c9392fa38
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>