Convert mkdtboimg.py to python 3 am: f20ea93eca am: 9cc2027ccc am: ca262c4ba6 am: 0734692e4d am: 0206b5ac34

Original change: https://android-review.googlesource.com/c/platform/system/libufdt/+/2061124

Change-Id: Ib89424838fa7acababb828d5f5851b8c47d0acb8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>