Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/scripts/gdb/Makefile b/scripts/gdb/Makefile
index 62f5f65..3fca193 100644
--- a/scripts/gdb/Makefile
+++ b/scripts/gdb/Makefile
@@ -1 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
 subdir-y := linux