Add support of more hash algorithms for verity tree builder

Currently, verified boot 1.0 is using SHA256 to compute the hash tree
while AVB is using SHA1. We should support at least these two hash
functionss in the HashTreeBuilder and command line parser. And we can
potentially add more algorithms in the future.

Bug: 25170618
Test: unit tests pass
Change-Id: I5f6dc8a545c0cef75acbbc2044959e8100f9f842
4 files changed