config: Make target argument optional in Config()

The target argument is only used when initializing the build plugin so
we can make it optional. This allows us to simplify the call to Config()
in the shell plugin and in other future plugins.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/plugins/shell.py

@@ -80,7 +80,7 @@ class Shell:
         """
 
         ctx = create_global_context(args)
-        ctx.config = Config(args.config, None, None)
+        ctx.config = Config(args.config)
 
         macro = Macro()