coreinfo: Unexport COREBOOT_EXPORTS

coreboot's Makefile exports a lot of variables that influence make sub-
processes (e.g. for Kconfig). We don't want these variables leak into
sub-processes for (lib)payload builds, hence unexport them.

Change-Id: I7d65c0aa6d4550bd6600c437e838339af69496da
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48939
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/coreinfo/Makefile

@@ -1,5 +1,7 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
+unexport $(COREBOOT_EXPORTS)
+
 src := $(CURDIR)
 srctree := $(src)
 srck := $(src)/../../util/kconfig