lib: sha256: move common function to sha256_common.c

lib: sha256: move common function to sha256_common.c

The function sha256_csum_wd is defined in lib/sha256.c
and in lib/mbedtls/sha256.c. To avoid duplicating this
function (and future function), we move this function
to the file lib/sha256_common.c

Reviewed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>