payloads/U-Boot: Unexport the coreboot_exports

Without this U-Boot ends up overwriting the coreboot .config when
generating a configfile.

Change-Id: I62fc0aa3ede0287ffc96915182a2ed5a4877f29d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32303
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

payloads/external/U-Boot/Makefile

@@ -30,6 +30,7 @@ unexport KCONFIG_DEPENDENCIES
 unexport KCONFIG_SPLITCONFIG
 unexport KCONFIG_TRISTATE
 unexport KCONFIG_NEGATIVES
+unexport $(COREBOOT_EXPORTS)
 
 all: build