Make mkdtboimg.py compatible with python3

There are a few changes needed to run mkdtboimg.py with python3.
-No more implicit conversion between char and byte.
-xrange was renamed to range in python3
-floor division must be done with the  '//' operator

Bug: 166496956
Test: ./mkdtboimg_testdata.sh
Test: atest FirmwareDtboVerification
Change-Id: I8b9b97b7b9f31768cc133f37d0144420cb1a0e81
1 file changed