Fix handling of `UKI/SBAT` and `UKI/Firmware` config entries. (#42478)

This fixes 2 issues with `ukify`:

`config_list_prepend` needs a list. The 2 more used entries are
specially treated (parsed into a list) and therefore happen to work, but
trying to set `UKI/Firmware` throws a type error, as it is passed as
just a string.

The `UKI/SBAT` config entry is a list but used the default `config_push`
value which sets it if unset (which also failed since its default of
`[]` is not `None`) - change this to `config_list_prepend`.

I did consider adding a `config_list_append`, but as far as I'm aware,
the order of sbat entries should not change the outcome anyway.