unpack_bootimg: fix pylint errors am: e71c440ea1 am: a17e34353f

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2044946

Change-Id: I04df3aaa13b536b6a02c27c582a75ad28d28a18a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>