Call clean before building xpis

The previous package target had `rm -f $$arch.xpi` but this
was never matching any XPI file built here.

Makefile

@@ -6,13 +6,19 @@ VERSION=`grep -Eo '"version"\:\s*"[^"]+' $(MANIFEST) | grep -Eo '[0-9].*'`
 define build-xpis
 	for arch in $(ARCHS); do \
 		echo "build $$arch-$(VERSION).xpi"; \
-		rm -f $$arch.xpi; \
 		zip $$arch-$(VERSION).xpi -r $$arch adb.json $(MANIFEST); \
 	done
 endef
 
+define clean
+	echo "Remove previous xpi files"; \
+	rm -f *.xpi
+endef
+
+# default target, clean previous XPIs and build new ones.
 package:
+	@$(call clean)
 	@$(call build-xpis)
 
 clean:
-	rm -f *.xpi
+	@$(call clean)