Make mkdtboimg.py compatible with python3 am: 2293727663 am: f5e4d426c1 am: 415d01437a am: 1f9e009223

Original change: https://android-review.googlesource.com/c/platform/system/libufdt/+/1415228

Change-Id: I47e58aeff89caf1a9b530c6bb87454c7cabb8458