Don't fail if one of the package lists expands to an empty list

When a package lists contains only packages protected by a test
that doesn't match for the current run, then Expand_package_list
outputs nothing and the following "grep -v" fails because it
has not filtered anything. Avoid this by protecting the "grep -v"
call with "|| true".

scripts/build/chroot_package-lists

@@ -71,7 +71,7 @@ do
 	then
 		# Generating package list
 		Expand_packagelist "$(basename ${LIST})" "config/package-lists" \
-			| grep -v '^#' >> chroot/root/packages.chroot
+			| grep -v '^#' >> chroot/root/packages.chroot || true
 	fi
 done