ukify: fix parsing of SignTool configuration option

ukify: fix parsing of SignTool configuration option

This partially reverts 02eabaffe98c9a3b5dec1c4837968a4d3e2ff7db.
As noted in https://github.com/systemd/systemd/pull/35211:
> The configuration parsing simply stores the string as-is, rather than
> creating the appropriate object

One way to fix the issue would be to store the "appropriate object", i.e.
actually the class. But that makes the code very verbose, with the conversion
being done in two places. And that still doesn't fix the issue, because we need
to map the class objects back to the original name in error messages.

So instead, store the setting as a string and only map it to the class much
later. This makes the code simpler and fixes the error messages too.

Resolves https://github.com/systemd/systemd/pull/35193