mkbootimg: add version and patch level

Adds operating system version and security patch level to the boot
image header. This change takes over two existing unused fields to
preserve compatibility with existing devices.

Bug: 27498078
Bug: 22914603
Change-Id: I57682611e0c27fec5ed284da7d9a788383ff98b1
(cherry pick from commit 3edc43234a0c0d592bc67b61f39c45eed238cef2)