kconfig: change defconfig_list option to environment variable

"defconfig_list" is a weird option that defines a static symbol that
declares the list of base config files in case the .config does not
exist yet.

This is quite different from other normal symbols; we just abused the
"string" type and the "default" properties to list out the input files.
They must be fixed values since these are searched for and loaded in
the parse stage.

It is an ugly hack, and should not exist in the first place. Providing
this feature as an environment variable is a saner approach.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/tests/conftest.py

@@ -53,6 +53,10 @@ class Conf:
         # Override 'srctree' environment to make the test as the top directory
         extra_env['srctree'] = self._test_dir
 
+        # Clear KCONFIG_DEFCONFIG_LIST to keep unit tests from being affected
+        # by the user's environment.
+        extra_env['KCONFIG_DEFCONFIG_LIST'] = ''
+
         # Run Kconfig in a temporary directory.
         # This directory is automatically removed when done.
         with tempfile.TemporaryDirectory() as temp_dir: