attendedsyuspgrade-common: add key and set server

attendedsyuspgrade-common: add key and set server

In collaboration with @dangowrt the server makes use of `ucert`.  Active
workers sign created firmware and clients check if the signature is
valid. Certs of *hacked* or inactive workers can be revoked.  Private CA
key is **not** stored on the upgrade server.

Only for devices already supporting ucert via firmware metadata.

Signed-off-by: Paul Spooren <redacted>