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 ifup@.service 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).