stubdom: fix make clean and distclean on a freshly cloned tree

stubdom: fix make clean and distclean on a freshly cloned tree

Clean and distclean targets need not depend on existence of the mini-os
tree. Don't check for mini-os and don't try to blindly include
mini-os's Config.mk when doing clean and distclean.

Note that one subtle issue is that $(XEN_ROOT)/Config.mk tries to
include $(XEN_ROOT)/config/$(XEN_OS).mk. In stubdom's case XEN_OS is
"MiniOS". Then $(XEN_ROOT)/config/MiniOS.mk tries to include mini-os's
Config.mk.

Since clean and distclean don't enforce existence of mini-os tree, don't
include $(XEN_ROOT)/Config.mk to avoid getting error due to the
aforementioned issue.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>