fs: btrfs: Add more checksum algorithms

This mostly crossports crypto/hash.[ch] from btrfs-progs.

The differences are:
- No blake2 support
  No blake2 related library in U-Boot yet.

- Use uboot xxhash/sha256 directly
  No need to implement the code as U-Boot has already provided the
  interface.

This adds the support for the following csums:
- SHA256
- XXHASH

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>