bootutil/crypto: Add a generic signature validation module for ECDSA

Add a dedicated signature validation module for generic ECDSA signatures,
and a corresponding cryptographic abstraction backend based on PSA Crypto
APIs. This signature verification backend is enabled by defining the
option MCUBOOT_SIGN_ECDSA

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: I47da70629da0a5681ec7c4dcceed875a997b071b