bootutil/crypto: SHA256 init functions should return a status

SHA-256 init functions should return the status of the init
instead of being void. This would allow the callers to implement
proper error recovery, otherwise on error the SHA-256 operation
will enter an undefined behaviour.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: I162ceb8e6dc90dc3c6b83c8a85fbd17b41c0b5d6