Kconfig: Move rustccfg file to a more benign place

It's put in $(obj) now. Not sure if we'll need it, but there has been
some interest in rust support in coreboot, and removing support for it
would be more work than this, so let's just keep it around.

Change-Id: I532fde9625dbf7463752ef1af525b77d12676c93
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79342
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

Makefile

@@ -23,6 +23,7 @@ COREBOOT_EXPORTS += top src srck obj objutil objk
 DOTCONFIG ?= $(top)/.config
 KCONFIG_CONFIG = $(DOTCONFIG)
 KCONFIG_AUTOADS := $(obj)/cb-config.ads
+KCONFIG_RUSTCCFG := $(obj)/cb-config.rustcfg
 KCONFIG_AUTOHEADER := $(obj)/config.h
 KCONFIG_AUTOCONFIG := $(obj)/auto.conf
 KCONFIG_DEPENDENCIES := $(obj)/auto.conf.cmd
@@ -42,6 +43,7 @@ COREBOOT_EXPORTS += KCONFIG_WERROR
 endif
 COREBOOT_EXPORTS += KCONFIG_WARN_UNKNOWN_SYMBOLS
 COREBOOT_EXPORTS += KCONFIG_AUTOADS KCONFIG_PACKAGE
+COREBOOT_EXPORTS += KCONFIG_RUSTCCFG
 
 # Make does not offer a recursive wildcard function, so here's one:
 rwildcard=$(wildcard $1$2) $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2))