Add a function sha256() which computes a SHA-256 digest in one step,
combining sha256_init() + sha256_update() + sha256_final().

This is similar to how we also have blake2s().
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>