kconfig: fix a bug of "make config"

Since 3ff291f371
(kconfig: convert Kconfig helper script into a shell script),
"make config" is not working because of a missing '$' before '(Q)'.

Besides, "make config" should be invoked via scripts/multiconfig.sh
to avoid a warning message:
Kconfig:11:warning: environment variable KCONFIG_OBJDIR undefined

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -458,7 +458,7 @@ KBUILD_DEFCONFIG := sandbox_defconfig
 export KBUILD_DEFCONFIG KBUILD_KCONFIG
 
 config: scripts_basic outputmakefile FORCE
-	(Q)$(MAKE) $(build)=scripts/kconfig $@
+	+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/multiconfig.sh $@
 
 %config: scripts_basic outputmakefile FORCE
 	+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/multiconfig.sh $@