crypto: sha256_generic - Switch to the generic lib/crypto/sha256.c lib code

Drop the duplicate generic sha256 (and sha224) implementation from
crypto/sha256_generic.c and use the implementation from
lib/crypto/sha256.c instead.

"diff -u lib/crypto/sha256.c sha256_generic.c" shows that the core
sha256_transform function from both implementations is identical and
the other code is functionally identical too.

Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>