build: rework .banner generation

Avoid depending on Makefile but still allow to rebuild the banner when
$(XEN_FULLVERSION) changes.

Also add a dependency on tools/xen.flf, even if not expected to
change.

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

xen/Makefile

@@ -378,13 +378,19 @@ delete-unfresh-files:
 		rm -f include/xen/compile.h; \
 	fi
 
-.banner: Makefile
-	@if which figlet >/dev/null 2>&1 ; then \
-		echo " Xen $(XEN_FULLVERSION)" | figlet -f tools/xen.flf > $@.tmp; \
-	else \
-		echo " Xen $(XEN_FULLVERSION)" > $@.tmp; \
-	fi
-	@mv -f $@.tmp $@
+quiet_cmd_banner = BANNER  $@
+define cmd_banner
+    if which figlet >/dev/null 2>&1 ; then \
+	echo " Xen $(XEN_FULLVERSION)" | figlet -f $< > $@.tmp; \
+    else \
+	echo " Xen $(XEN_FULLVERSION)" > $@.tmp; \
+    fi; \
+    mv -f $@.tmp $@
+endef
+
+.banner: tools/xen.flf FORCE
+	$(call if_changed,banner)
+targets += .banner
 
 # compile.h contains dynamic build info. Rebuilt on every 'make' invocation.
 include/xen/compile.h: include/xen/compile.h.in .banner