kconfig: move default KBUILD_DEFCONFIG back to scripts/kconfig/Makefile

This is a partial revert of commit 2a86f661

 ("kbuild: use
KBUILD_DEFCONFIG as the fallback for DEFCONFIG_LIST").

Now that the reference to $(DEFCONFIG_LIST) was removed from
init/Kconfig, the default KBUILD_DEFCONFIG can go back home.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>