bootutil/crypto: Builtin ECDSA key support for PSA Crypto backend

Enable the usage of builtin keys in the ECDSA verification module with
the PSA Crypto API based cryptographic backend.
This way parsing and importing the verification keys can also be avoided.

Change-Id: I6ada1ef8ed04a3f12c228ef399e3a7b8ebc7fb5e
Signed-off-by: David Vincze <david.vincze@arm.com>