Add make clean to support cleaning the rootfs and ubuntu.tar.
Add make clean to support cleaning the rootfs and ubuntu.tar.

Signed-off-by: LinZhinan(Zen Lin) <dcaf8624321f9dc0e5d38c2534fb14e619ae2d49@huawei.com>

wangkirin/oct,wking/rkt-oci,liangchenye/oct,huawei-openlab/oct,zenlinTechnofreak/oct