build: Make XEN_ROOT an absolute path.

Otherwise make can search the path relative to certain standard paths
such as /usr/include (e.g., the line '-include $(XEN_ROOT)/.config' in
Config.mk suffers from this).

Signed-off-by: Keir Fraser <keir@xen.org>

xen-unstable changeset: 23049:ff3b7749008b
Backport-requested-by: Allen M Kay <allen.m.kay@intel.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/hotplug/common/Makefile

@@ -1,4 +1,4 @@
-XEN_ROOT = ../../..
+XEN_ROOT = $(CURDIR)/../../..
 include $(XEN_ROOT)/tools/Rules.mk
 
 HOTPLUGPATH="hotplugpath.sh"