mkbootimg: Add AsciizBytes argument parser

AsciizBytes() parses a string argument and encodes it as an asciiz bytes.
The encoded bytes object is ensured to have a null-terminator.

Clarify in bootimg.h that c-string (text) fields are actually asciiz.

Bug: 180792243
Bug: 180712476
Test: python3 -m doctest -v mkbootimg.py
Test: atest --host mkbootimg_test
Change-Id: I0ab6e6b3835225b619c3b4bea101b921ed1c7cbd