unpack_bootimg: fix pylint errors am: e71c440ea1

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

Change-Id: I0e35488cb264d9f3e064fea63db0530250233428
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>