Fix buildroot clean not work

[demin-han]

These three clean not work:

1. with wildcards, shell=True should be passed. But if delete overlay/* really, build will fail because of missing etc/init.d/S99run. It's not necessary to clean overlay, it's overwritten every time.

2. with wildcards, shell=True should be passed. We can delete target directly

3. '".stamp_target_installed"' not match our expected.