Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/tools/bpf/Makefile.helpers b/tools/bpf/Makefile.helpers
index c34fea7..854d084 100644
--- a/tools/bpf/Makefile.helpers
+++ b/tools/bpf/Makefile.helpers
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 ifndef allow-override
   include ../scripts/Makefile.include
   include ../scripts/utilities.mak