kunit: use --build_dir=.kunit as default

To make KUnit easier to use, and to avoid overwriting object and
.config files, the default KUnit build directory is set to .kunit

 * Related bug: https://bugzilla.kernel.org/show_bug.cgi?id=205221

Fixed up minor merge conflicts - Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>