zoneconf.cgi: Avoid unnecessary MAC address changes

zoneconf.cgi: Avoid unnecessary MAC address changes

Ensure that a bridge always has a MAC address configured, to prevent
udev/network-hotplug-bridges assigning random addresses at each start.
Cache previously generated MAC addresses so that they are not
regenerated each time the configuration is saved by the user.

Add more comments to existing code.

Fixes: #12583
Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>