ukify: deal with non-list values in config_list_prepend

`ConfigItem.config_list_prepend` assumed that the passed value was a
list. This just happens to be the case for the `--initrd` and
`--devicetree-auto` parameters, as those get special treatment in
`ConfigItem.apply_config` as they use space separated lists.

However, this is not generally the case: When trying to set
`UKI/Firmware` the following error occurred:

Traceback (most recent call last):
  File "/usr/bin/ukify", line 2565, in <module>
    main()
    ~~~~^^
  File "/usr/bin/ukify", line 2548, in main
    opts = UkifyConfig.from_namespace(parse_args())
                                      ~~~~~~~~~~^^
  File "/usr/bin/ukify", line 2542, in parse_args
    apply_config(opts)
    ~~~~~~~~~~~~^^^^^^
  File "/usr/bin/ukify", line 2295, in apply_config
    item.apply_config(namespace, section_name, group, key, value)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/bin/ukify", line 1892, in apply_config
    self.config_push(namespace, group, dest, value)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/bin/ukify", line 1765, in config_list_prepend
    setattr(namespace, dest, value + old)
                             ~~~~~~^~~~~
TypeError: unsupported operand type(s) for +: 'PosixPath' and 'list'

Since we can thus get both lists and strings, check the type and
convert non-list values to `[value]`.

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

src/ukify/ukify.py

@@ -1762,6 +1762,8 @@ class ConfigItem:
         old = getattr(namespace, dest, [])
         if old is None:
             old = []
+        if not isinstance(value, list):
+            value = [value]
         setattr(namespace, dest, value + old)
 
     @staticmethod