Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/arm/mach-stm32/Makefile.boot b/arch/arm/mach-stm32/Makefile.boot
index eacfc3f..cec195d 100644
--- a/arch/arm/mach-stm32/Makefile.boot
+++ b/arch/arm/mach-stm32/Makefile.boot
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 # Empty file waiting for deletion once Makefile.boot isn't needed any more.
 # Patch waits for application at
 # http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=7889/1 .