bootutil: Refactor signature verification

The current ECDSA-P256 implementation code contains
a lot of code that is tied to a specific condition being met.
The aim of this commit is to cleanup the main verification
logic to be unified between crypto backends and move the
conditional code where it is relevant.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I06b050a263b2b88b08708defb6aa1001a08ba2ae