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>

1 files changed, 3 insertions, 2 deletions

diff --git a/include/crypto/sha256.h b/include/crypto/sha256.h
index b1f9c6781082..9cbb3589b8b3 100644
--- a/include/crypto/sha256.h
+++ b/include/crypto/sha256.h
@@ -14,8 +14,9 @@
 /*
  * Stand-alone implementation of the SHA256 algorithm. It is designed to
  * have as little dependencies as possible so it can be used in the
- * kexec_file purgatory. In other cases you should use the implementation in
- * crypto/.
+ * kexec_file purgatory. In other cases you should generally use the
+ * hash APIs from include/crypto/hash.h. Especially when hashing large
+ * amounts of data as those APIs may be hw-accelerated.
  *
  * For details see lib/crypto/sha256.c
  */