Modify mkbootimg.py to support boot image header version 2

Boot image with header version 2 supports inclusion of
DTB image within the boot image.

Bug: 111136242
Test: unpack_bootimg.py is able to extract DTB added by
mkbootimg.py

Change-Id: Ie6ae2507aa0c1960c73077d618aacafb83ae8459
3 files changed