qibuild clean: be obvious, clean deep by default, respect -c, -s, proj

Change-Id: Ib663819d109d2b206494150799c433aec1d5ddce
Reviewed-on: http://gerrit.aldebaran.lan:8080/10074
Reviewed-by: ncornu <ncornu@aldebaran-robotics.com>
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Tested-by: ckilner <ckilner@aldebaran-robotics.com>