kunit: Create default config in '--build_dir'

If both '--build_dir' and '--defconfig' are given, the handling of
'--defconfig' ignores '--build_dir' option.  This commit modifies the
behavior to respect '--build_dir' option.

Reported-by: Brendan Higgins <brendanhiggins@google.com>
Suggested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>