util: Add SPDX license headers to Makefiles

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I7cf35132df0bc23f7b6f78014ddd72d58ea2ab8a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68983
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

util/futility/Makefile

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 top ?= $(abspath ../..)
 objutil ?= $(top)/util
 RM ?= rm

util/futility/Makefile.inc

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 additional-dirs += $(objutil)/futility
 
 VBOOT_FUTILITY = $(VBOOT_HOST_BUILD)/futility/futility