certify_bootimg: avoid using subprocess calls to unpack_bootimg am: ac2f0c28b2

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

Change-Id: I237d38f165a14df38c1c4a9dbafccfacea5a46bb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>