Add Assert statements to crypto calls in MCUboot

[Karambite]

I'm open to any discussion, but I think there should be asserts in instances like this.

When porting a unique crypto library, it is understood that the developer should handle errors, however, in mcuboot, an error doesn't end the mcuboot hash operation early. Instead it still continues till the hash check is incorrect. An assert here to make sure that if there are errors (like failed crypto engine boot) and end mcuboot crypto operation end earlier would be an improvement.