mkbootimg: add os_version

Adds operating system version and security patch level to the boot
image header. This change take into use one of the existing unused
fields to preserve compatibility.

Note: All information is packed into a single field, because there
are a lot of existing devices that break if the first unused field
is set to a non-zero value.

Bug: 27498078
Bug: 22914603
Change-Id: I24953129e06019b95014a050e916fe4f5c199286