bootutil/crypto: SHA256 abort function return state

Similarly to what has been done for the init function, also
the abort function should return a state in case the caller
needs to implement some error recovery procedure, or even
just for debugging reasons.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: I5b8bc8fc2da57cfbc6ddea3f7e95ed7a7ae8e5a9