kbuild: fix interaction of CONFIG_IKCONFIG and KCONFIG_CONFIG

If you try to build a kernel with KCONFIG_CONFIG set (to a value
not equal to .config) and that config sets CONFIG_IKCONFIG then the
build will fail with:

make[1]: *** No rule to make target `.config', needed by \
`kernel/config_data.gz'.  Stop.

because the kernel/Makefile contains a direct reference to .config.

This issue has been present since the introduction of KCONFIG_CONFIG
in 14cdd3c4.
Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Roman Zippel <zippel@linux-m68k.org>
CC: Michal Marek <mmarek@suse.cz>
Reviewed-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -224,6 +224,7 @@ ifeq ($(ARCH),m68knommu)
 endif
 
 KCONFIG_CONFIG	?= .config
+export KCONFIG_CONFIG
 
 # SHELL used by kbuild
 CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \

kernel/Makefile

@@ -121,7 +121,7 @@ $(obj)/configs.o: $(obj)/config_data.h
 # config_data.h contains the same information as ikconfig.h but gzipped.
 # Info from config_data can be extracted from /proc/config*
 targets += config_data.gz
-$(obj)/config_data.gz: .config FORCE
+$(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
 	$(call if_changed,gzip)
 
 quiet_cmd_ikconfiggz = IKCFG   $@