certify_bootimg: makes gki-info.txt optional am: 79627361ad am: 6236378e71

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

Change-Id: Iec7c84a2c6beaac55ceef1ed7f3a4a89fbcc448e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>