fix build with CONFIG_HYPFS_CONFIG enabled

Commit 58263ed771 ("xen: add /buildinfo/config entry to hypervisor
filesystem") added a dependency to .config, but the hypervisor's build
config could be have another name via setting KCONFIG_CONFIG.

Fix that by using $(KCONFIG_CONFIG) instead. Additionally reference
the config file via $(XEN_ROOT) instead of a relative path.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/Makefile

@@ -75,7 +75,8 @@ obj-$(CONFIG_UBSAN) += ubsan/
 obj-$(CONFIG_NEEDS_LIBELF) += libelf/
 obj-$(CONFIG_HAS_DEVICE_TREE) += libfdt/
 
-config.gz: ../.config
+CONF_FILE := $(if $(patsubst /%,,$(KCONFIG_CONFIG)),$(XEN_ROOT)/xen/)$(KCONFIG_CONFIG)
+config.gz: $(CONF_FILE)
 	gzip -c $< >$@
 
 config_data.o: config.gz