Support certifying the boot.img in a boot-img.tar.gz am: 90a8372549

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2109068

Change-Id: I20aec7972adb225b6b2aa495a8aedc2af9e2e085
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>