Hi, I've recently bought a Solokey, which implements FIDO2 as well and I have been using it on my router for a while now.
Since OpenSSH is already on 8.4p on master, I've opened a PR to add support for all FIDO2 keys on the official OpenWRT openssh-server package.
Also, a few notes: libsk-libfido2 is not required anymore by OpenSSH anymore.
All that's needed is libcbor, libfido2 itself and OpenSSH to be compiled with the --with-security-key-builtin flag.
If anyone wants to compile their own openssh-server package, feel free to use this PR. Hopefully it'll get merged at some point and it'll probably land on the next OpenWRT release.
Instead of building your own OpenWRT image from scratch, you can just build the openssh-server package using the compiled OpenWRT SDK for your current version. You can find more information here