Prevent a buffer overflow in mkimage when signing with SHA256

Prevent a buffer overflow in mkimage when signing with SHA256

Due to the FIT_MAX_HASH_LEN constant not having been updated
to support SHA256 signatures one will always see a buffer
overflow in fit_image_process_hash when signing images that
use this larger hash.  This is exposed by vboot_test.sh.

Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Acked-by: Simon Glass <sjg@chromium.org>
[trini: Rework a bit so move the exportable parts of hash.h outside of
 !USE_HOSTCC and only need that as a new include to image.h]
Signed-off-by: Tom Rini <trini@ti.com>