add xz params to tighten archive size (saves 5M currently), use all available...

add xz params to tighten archive size (saves 5M currently), use all available CPU cores, overwrite existing file
Signed-off-by: Linux Ninja <linuxninja@bryangay.com>

Makefile

@@ -20,7 +20,7 @@ rootfs: hooks
 	rm -rf $(BUILDDIR) alpm-hooks
 
 compress-rootfs: rootfs
-	xz archlinux.tar
+	xz -9e -T0 -f archlinux.tar
 
 docker-image: compress-rootfs
 	docker build -t $(DOCKER_ORGANIZATION)/$(DOCKER_IMAGE) .