kconfig: fix a bug of "make config"

Since 3ff291f3
(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>