Calculate the vbmeta digest when building images

Calculate the vbmeta digest if the device builds vbmeta image. The
digest will used later to determine the build fingerprint in new
format.

One sample usage is the ota package generation, where we put the
build fingerprint in the ota metadata. But we don't have the runtime
vbmeta digest provided the bootloader.

Bug: 186786987
Test: unit tests
Change-Id: If572e2b973e295a6c95a9e23a65bb20b3afbf1b0