x509: Add support for parsing x509 certs with ECDSA keys

Add support for parsing of x509 certificates that contain ECDSA keys,
such as NIST P256, that have been signed by a CA using any of the
current SHA hash algorithms.

Cc: David Howells <dhowells@redhat.com>
Cc: keyrings@vger.kernel.org
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>