This updates the generic SHA-256 implementation to use the
new shared SHA-256 glue code.

It also implements a .finup hook crypto_sha256_finup() and exports
it to other modules. The import and export() functions and the
.statesize member are dropped, since the default implementation
is perfectly suitable for this module.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>