kconfig: make arch/*/configs/defconfig the default of KBUILD_DEFCONFIG

Until recently, if KBUILD_DEFCONFIG was not set by the arch Makefile,
the default path arch/*/defconfig was used.

The last users of the default are gone by the following commits:

- Commit f3e20ad67b ("s390: move arch/s390/defconfig to
  arch/s390/configs/defconfig")

- Commit 986a13769c ("alpha: move arch/alpha/defconfig to
  arch/alpha/configs/defconfig")

Let's set arch/*/configs/defconfig as a new default. This saves
KBUILD_DEFCONFIG for some architectures.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>

arch/nds32/Makefile

@@ -2,8 +2,6 @@
 LDFLAGS_vmlinux	:= --no-undefined -X
 OBJCOPYFLAGS	:= -O binary -R .note -R .note.gnu.build-id -R .comment -S
 
-KBUILD_DEFCONFIG := defconfig
-
 ifdef CONFIG_FUNCTION_TRACER
 arch-y += -malways-save-lp -mno-relax
 endif