RCF: [ifupdown] Fix bridges down at boot with Debian bookworm systemd

[prahal]

With Debian bookworm systemd 252.6-1, the bridge is stopped immediately after attempting its startup.
It could be that this newer systemd removed the unexpected behavior that allowed to start the service while its "BindsTo" device does not exists.

Workaround for #2370 .

[prahal]

@drybjed Note that I cooked this workaround without a reply to my request in #2370 for you to explain me the idea you told about on reddit
https://www.reddit.com/r/debian/comments/7nw98g/comment/ds57s74/ "stick to ifupdown and [email protected] but configure virtual network interfaces as needed in udev beforehand. I'm not sure how this should be done properly, maybe with aid of .device units that define these devices on boot. But this involves more moving parts and I didn't really check if that solution is viable."
I don't think one can create an interface with udev without binding it to a physical interface event (add, change, remove). So to me udev cannot help to create this bridge device before running iface@ service.
We could ask the systemd projects devs how they envision this bridge device creation (or if they have other ideas to cope with this issue).

[drybjed]

This is probably a good workaround for current issues, I hope that you tested this. :)

The solution to the issue you mentioned is to create bridge devices in udev ourselves, yes. I'm not sure how ifupdown developers handle this in newer Debian releases, but in networkd it's just a few configuration options (see examples in the networkd role documentation). When the ifupdown role is refreshed this probably will be taken care of in the way ifupdown service does it. In simpler setups, switching to networkd is probably an easier way than to use ifupdown.