Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index c230c2d..2efe1e2 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Common SMP CPU bringup/teardown functions
  */