ukify: honor UKI/SBAT from config files

`UkifyConfig.sbat` is an optional list, so the default of
`config_set_if_unset` would only accept a single value.

Further, it is defined with an empty list as a default, causing
configuration values to always be ignored, as the default is not
`None`.

Set its `config_push` to `ConfigItem.config_list_prepend` to collect
the `SBAT` lines from the config.

Signed-off-by: Wolfgang Bumiller <w.bumiller@errno.eu>

src/ukify/ukify.py

@@ -2018,6 +2018,7 @@ CONFIG_ITEMS = [
         default=[],
         action='append',
         config_key='UKI/SBAT',
+        config_push=ConfigItem.config_list_prepend,
     ),
     ConfigItem(
         '--pcrpkey',