lib: add BLAKE2 hash support

The code is cross-ported from BLAKE2 reference implementation
(https://github.com/BLAKE2/BLAKE2

).

With minimal change to remove unused macros/features.

Currently there is only one user inside U-boot (btrfs), and since it
only utilize BLAKE2B, all other favors are all removed.

Signed-off-by: Qu Wenruo <wqu@suse.com>