certify_bootimg: support --gki_info for a boot.img am: 3886f4a68b

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2079119

Change-Id: Idf95fe69a345cb785cc83c32726c2e24e549b1b2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>