certify_bootimg: avoid using subprocess calls to unpack_bootimg am: ac2f0c28b2 am: f149ca0d9a

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2038863

Change-Id: Ia4f5cc46428300560e1ee82401b555e1fc40920d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>