crypto: sha256 - Make lib/crypto/sha256.c suitable for generic use

crypto: sha256 - Make lib/crypto/sha256.c suitable for generic use

Before this commit lib/crypto/sha256.c has only been used in the s390 and
x86 purgatory code, make it suitable for generic use:

* Export interesting symbols
* Add  -D__DISABLE_EXPORTS to CFLAGS_sha256.o for purgatory builds to
  avoid the exports for the purgatory builds
* Add to lib/crypto/Makefile and crypto/Kconfig

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