[x509] Record root of trust used when validating a certificate

Record the root of trust used at the point that a certificate is
validated, redefine validation as checking a certificate against a
specific root of trust, and pass an explicit root of trust when
creating a TLS connection.

This allows a custom TLS connection to be used with a custom root of
trust, without causing any validated certificates to be treated as
valid for normal purposes.

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

src/net/tcp/https.c

@@ -46,7 +46,7 @@ FEATURE ( FEATURE_PROTOCOL, "HTTPS", DHCP_EB_FEATURE_HTTPS, 1 );
  */
 static int https_filter ( struct http_connection *conn ) {
 
-	return add_tls ( &conn->socket, conn->uri->host );
+	return add_tls ( &conn->socket, conn->uri->host, NULL );
 }
 
 /** HTTPS URI opener */