sha2: add implementation of the sha256 digest algo

Add a minimal implementation of the SHA-256 digest algorithm.
This implementation is pulled from the OpenBSD project, and [1] [2]
were used as a base.

[1] https://github.com/openbsd/src/blob/master/lib/libc/hash/sha2.c
[2] https://github.com/openbsd/src/blob/master/include/sha2.h

Signed-off-by: Igor Opaniuk <igor.opaniuk@oss.qualcomm.com>

Author: igoropaniuk