kconfig: Use KCONFIG_CONFIG instead of .config

When using a custom location for kernel config files this merge config
command fails as it doesn't use the configuration set with
KCONFIG_CONFIG.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/Makefile

@@ -107,7 +107,7 @@ config-fragments = $(call configfiles,$@)
 
 %.config: $(obj)/conf
 	$(if $(config-fragments),, $(error $@ fragment does not exists on this architecture))
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m .config $(config-fragments)
+	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m $(KCONFIG_CONFIG) $(config-fragments)
 	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
 PHONY += tinyconfig