crypto: sha256 - Move lib/sha256.c to lib/crypto

crypto: sha256 - Move lib/sha256.c to lib/crypto

Generic crypto implementations belong under lib/crypto not directly in
lib, likewise the header should be in include/crypto, not include/linux.

Note that the code in lib/crypto/sha256.c is not yet available for
generic use after this commit, it is still only used by the s390 and x86
purgatory code. Making it suitable for generic use is done in further
patches in this series.

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