build: set policy filename on make command line

In order to avoid flask/Makefile.common calling `make xenversion`, we
override POLICY_FILENAME with the value we are going to use anyway.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/xsm/flask/Makefile

@@ -42,7 +42,9 @@ FLASK_BUILD_DIR := $(CURDIR)
 POLICY_SRC := $(FLASK_BUILD_DIR)/xenpolicy-$(XEN_FULLVERSION)
 
 policy.bin: FORCE
-	$(MAKE) -f $(XEN_ROOT)/tools/flask/policy/Makefile.common -C $(XEN_ROOT)/tools/flask/policy FLASK_BUILD_DIR=$(FLASK_BUILD_DIR)
+	$(MAKE) -f $(XEN_ROOT)/tools/flask/policy/Makefile.common \
+	        -C $(XEN_ROOT)/tools/flask/policy \
+	        FLASK_BUILD_DIR=$(FLASK_BUILD_DIR) POLICY_FILENAME=$(POLICY_SRC)
 	cmp -s $(POLICY_SRC) $@ || cp $(POLICY_SRC) $@
 
 .PHONY: clean