only rebuild the package index when the *.ipk files have changed

git-svn-id: svn://svn.openwrt.org/openwrt/branches/buildroot-ng/openwrt@4395 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/Makefile

@@ -55,6 +55,8 @@ install:
 	rm -rf $(BUILD_DIR)/root
 	$(MAKE) install-targets
 
-index: FORCE
+index: $(PACKAGE_DIR)/Packages
+
+$(PACKAGE_DIR)/Packages: $(PACKAGE_DIR)/*.ipk
 	(cd $(PACKAGE_DIR); $(STAGING_DIR)/usr/bin/ipkg-make-index . > Packages)