certify_bootimg: support --extra_footer_args am: f754a7c3b8

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2104149

Change-Id: I8229a65beff1434e48162d499ee32e179ada9a85
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>