Fall back to .config.yaml if no configuration file given

Make the configuration file on the command line optional and fall back
to trying to open the generated .config.yaml instead. This allows the
sequence

kas menu -> save & exit
kas build
kas shell
kas ...

and also makes rebuilding the self-configured image simpler.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/plugins/menu.py

@@ -75,6 +75,7 @@ from kconfiglib import Kconfig, Symbol, Choice, expr_value, TYPE_TO_STR, \
     MENU, COMMENT, STRING, BOOL, INT, HEX, UNKNOWN
 from kas import __version__, __file_version__
 from kas.context import create_global_context
+from kas.config import CONFIG_YAML_FILE
 from kas.includehandler import load_config as load_config_yaml
 from kas.plugins.build import Build
 
@@ -90,8 +91,6 @@ __copyright__ = \
     'Copyright (c) 2011-2019, Ulf Magnusson <ulfalizer@gmail.com>\n' \
     'Copyright (c) Siemens AG, 2021'
 
-CONFIG_YAML_FILE = '.config.yaml'
-
 
 def check_sym_is_string(sym):
     if sym.type != STRING: