Makefile: release: use .tar.xz instead of .tar.bz2

xz compresses better than bzip2, and is (getting) more popular, so build
release tarballs as .tar.xz (in addition to .tar.gz) instead of .tar.bz2,
similar to how the kernel did ~8 years ago:

https://www.kernel.org/happy-new-year-and-good-bye-bzip2.html

-rw-r--r-- 1 peko peko 5,1M Dec  2 17:55 buildroot-2021.11-rc3.tar.xz
-rw-r--r-- 1 peko peko 5,7M Nov 30 18:15 buildroot-2021.11-rc3.tar.bz2
-rw-r--r-- 1 peko peko 6,8M Nov 30 18:15 buildroot-2021.11-rc3.tar.gz

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -1210,7 +1210,7 @@ release:
 	$(MAKE) O=$(OUT) distclean
 	tar rf $(OUT).tar $(OUT)
 	gzip -9 -c < $(OUT).tar > $(OUT).tar.gz
-	bzip2 -9 -c < $(OUT).tar > $(OUT).tar.bz2
+	xz -9 -c < $(OUT).tar > $(OUT).tar.xz
 	rm -rf $(OUT) $(OUT).tar
 
 print-version: