[build][arm64] Add support for Android Verified Boot (AVB)

Adds support for packaging zircon as an Android style boot image
and generating a vbmeta image for devices that use AVB.
Includes the minimal subset of Android verified boot needed for this
based on AOSP external/avb at:

https://android.googlesource.com/platform/external/avb/+/4f137c38f8386bfcd778bdfc798c6299affc0d20

TEST: Zircon boots on mt8167s_ref board with a CL to be provided separately.
Change-Id: I8db64a88bd9b0f566001f19d5f39c9d8b8b07406