[crypto] Remove dynamically-allocated storage for certificate name

iPXE currently allocates a copy the certificate's common name as a
string.  This string is used by the TLS and CMS code to check
certificate names against an expected name, and also appears in
debugging messages.

Provide a function x509_check_name() to centralise certificate name
checking (in preparation for adding subjectAlternativeName support),
and a function x509_name() to provide a name to be used in debugging
messages, and remove the dynamically allocated string.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/validator.c

@@ -179,7 +179,7 @@ static int validator_append ( struct validator *validator,
 		}
 		cert = x509_last ( certs );
 		DBGC ( validator, "VALIDATOR %p found certificate %s\n",
-		       validator, cert->subject.name );
+		       validator, x509_name ( cert ) );
 
 		/* Move to next certificate */
 		asn1_skip_any ( &cursor );