Makefile: release: create tarball from current branch, not master

To support stable bugfix releases.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -841,7 +841,7 @@ release: OUT=buildroot-$(BR2_VERSION)
 # Create release tarballs. We need to fiddle a bit to add the generated
 # documentation to the git output
 release:
-	git archive --format=tar --prefix=$(OUT)/ master > $(OUT).tar
+	git archive --format=tar --prefix=$(OUT)/ HEAD > $(OUT).tar
 	$(MAKE) O=$(OUT) manual-html manual-txt manual-pdf
 	tar rf $(OUT).tar $(OUT)
 	gzip -9 -c < $(OUT).tar > $(OUT).tar.gz