Broadcom wl module fails to insert after upgrade to 18.04 release

[James]

I upgraded to the 18.04 beta in early April, and everything worked. I've just done another dist-upgrade to the final release, and the Broadcom wl wireless driver fails to insert into the kernel, the following error is encountered:

Required key not available

I get this with the 4.15.0-20-generic kernel, booting into a 4.15.0-13-generic kernel works fine. Perhaps this change is related, the kernel is now signed by default:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1764794

If that is the cause - is there a better way that this could be handled without requiring a user to spend several hours researching the problem? Perhaps mention it in the release notes, with links to documentation on how to resolve if you are using unsigned modules?