atftp: include whole sysconfdir in PN

* when sysv script is removed QA warn is shown:
  atftp-0.7.1+gitAUTOINC+be3291a18c: atftp: Files/directories were installed but not shipped
    /etc
  empty dir doesn't cost much, so just include it

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>