kconfig: fix failing to generate auto.conf

When the KCONFIG_AUTOCONFIG is specified (e.g. export \
KCONFIG_AUTOCONFIG=output/config/auto.conf), the directory of
include/config/ will not be created, so kconfig can't create deps
files in it and auto.conf can't be generated.

Signed-off-by: Jing Leng <jleng@ambarella.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>