unpack_bootimg: fix pylint errors am: e71c440ea1 am: a17e34353f am: 884f5bb06e

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

Change-Id: I048a4465a1410f6395c33ea6a1e73dce28c724bb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>