payloads: Add SPDX headers to Kconfig

Change-Id: Iea569fd457b3cd1f4746fbc6a96319eb42733a6b
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80586
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

payloads/external/linux/Kconfig.name

@@ -1,3 +1,5 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
 config PAYLOAD_LINUX
 	bool "A Linux payload"
 	depends on ARCH_X86 || ARCH_ARM || ARCH_RISCV