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

This is a partial revert of commit 2a86f66121 ("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>

scripts/kconfig/Makefile

@@ -9,6 +9,10 @@ else
 Kconfig := Kconfig
 endif
 
+ifndef KBUILD_DEFCONFIG
+KBUILD_DEFCONFIG := defconfig
+endif
+
 ifeq ($(quiet),silent_)
 silent := -s
 endif