ukify: raise error if genkey is called with no output arguments

ukify: raise error if genkey is called with no output arguments

The idea is that genkey is called with either
--secureboot-private-key= + --secureboot-certificate=, and then it
writes those, or with --pcr-private-key + optionally --pcr-public-key
and then it writes those, or both. But when called with no arguments
whatsover, it did nothing.

There is no implicit value for any of those parameters as input (unlike in
mkosi), so we also don't want to have implicit values when used as output.
But we shouldn't return success if no work was done, this is quite confusing.