Support blake2b-256 in hashtree builder

Now that blake2b-256 can be used to generate hashtree, add
the algorithm support in hashtree builder. This is needed to
perform on-device verity computation during an OTA update.

Bug: 178117377
Test: disable vabc on CF, check blake2b-256 verity tree generates correctly on device
Change-Id: I54de6f9db34f7b128f914de15af9b7fb61c41d1a
2 files changed