bootutil/crypto: Have a single ECDSA verification module

Remove the generic ECDSA verification module and keep the
existing one, just renaming it image_ecdsa.c. Make sure
that the abstraction layer is generically called ecdsa.h
and the abstraction names are not P256 specific.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: I6f78cfc1b1c2851cdad67efa91c6cb49498187bb