crypto: sha256 - implement base layer for SHA-256

To reduce the number of copies of boilerplate code throughout
the tree, this patch implements generic glue for the SHA-256
algorithm. This allows a specific arch or hardware implementation
to only implement the special handling that it needs.

The users need to supply an implementation of

  void (sha256_block_fn)(struct sha256_state *sst, u8 const *src, int blocks)

and pass it to the SHA-256 base functions. For easy casting between the
prototype above and existing block functions that take a 'u32 state[]'
as their first argument, the 'state' member of struct sha256_state is
moved to the base of the struct.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>