ukify: add --pcr-certificate= parameter

ukify: add --pcr-certificate= parameter

Public keys and certificates are not the same, as the latter embeds more
information that the former, and other tools like sd-measure have distinct
parameters for each of them.
Add a new --pcr-certificate= parameter to ukify, and use it to pass certs
down to sd-measure, as an alternative to --pcr-public-key=. Do not allow
specifying both.