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 <mail@aparcar.org>

due to various clients and the existence of rpcd-mod-rpcsys the package
rpcd-mod-attendedsysupgrade is superflous.

From now on the attendedsysupgrade-common package should be dependencie
of all kinds of clients.

Currently active or planned:
- luci-app-attendedsysupgrade
- attendedsysupgrade cli in shell
- attendedsysupgrade cli in c

Signed-off-by: Paul Spooren <paul@spooren.de>