xen/build: fixup path to merge_config.sh

This resolves the following observed error:

/bin/sh: /path/to/xen/xen/../xen/scripts/kconfig/merge_config.sh: No such file or directory

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

xen/tools/kconfig/Makefile

@@ -93,7 +93,7 @@ configfiles=$(wildcard $(srctree)/kernel/configs/$@ $(srctree)/arch/$(SRCARCH)/c
 
 %.config: $(obj)/conf
 	$(if $(call configfiles),, $(error No configuration exists for this target on this architecture))
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m .config $(configfiles)
+	$(Q)$(CONFIG_SHELL) $(srctree)/tools/kconfig/merge_config.sh -m .config $(configfiles)
 	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
 PHONY += kvmconfig