Convert mkdtboimg.py to python 3 am: f20ea93eca am: 9cc2027ccc am: ca262c4ba6

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

Change-Id: I4e9a155dc46f747ae54d7785e85e8ac83b443d2c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>