Populate header size for boot image header version 2 correctly

Bootimage header size for header version 2 was being populated
incorrectly. Also, throw an error when an unsupported boot header
version is passed in as an argument.

Test: can see correct header size using unpack_bootimg.py
Bug: 111136242
Change-Id: Ia4ed7b2469a5296546627c91f441c496d67f5ce3