libavb: Add new routine to calculate a digest of all vbmeta images.

This is similar to the androidboot.vbmeta.{digest, hash_alg, size}
properties set on the kernel commandline. The intent is to make this
data easily available to the bootloader without having it parse the
commandline.

Bug: None
Test: Update unit tests and all unit tests pass.
Change-Id: I4134f1a4116468bf063db2f15b8b67edd26918c2