Image builds create a key separate from the SecureBoot one for verity
signing. Its certificate ships inside the image as the sysext trust
anchor.
`genkey` certs had a hardcoded "SecureBoot signing key on host <fqdn of
the build machine>" CN, which describes neither that verity role nor a
meaningful hostname.
Make that name configurable with `SecureBootCertificateCommonName=`,
like mkosi's `--genkey-common-name`. That allows image build tools to
re-use `ukify genkey` for non-SB keys.
Unlike the implicit truncation for the default name, a specified one
which is too long is an error. Add a citation where that limit comes
frm.
Martin Pitt
b8bcae41ac