Convert mkdtboimg.py to python 3 am: f20ea93eca

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

Change-Id: I2cddc2a5b442d3bd82db9bfaaffe5b8aa8959ced
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>