build system: Exempt make *config from strict symbol checks

The "config" targets exist to edit the .config file, and so they
should be more forgiving with invalid configs (that they'll convert
into valid configs on save). They will still emit warnings about
invalid symbols, but not exit with an error.

The regular build process still fails if the .config looks unexpected
(for example when there's an unknown config flag).

Change-Id: If427e075766c68d493dd406609f21b6bb27d1d74
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79298
Reviewed-by: Mike Banon <mikebdp2@gmail.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

util/kconfig/Makefile.inc

@@ -47,8 +47,6 @@ unexport KCONFIG_DEFCONFIG_LIST
 
 # Fill in Linux kconfig build rules to work
 
-oldconfig: KCONFIG_STRICT=
-
 savedefconfig: $(objk)/conf
 	cp $(DOTCONFIG) $(DEFCONFIG)
 	chmod +w $(DEFCONFIG)