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 f3e20ad67b4c ("s390: move arch/s390/defconfig to
arch/s390/configs/defconfig")

- Commit 986a13769c4b ("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 <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>

arch/alpha/Makefile

···
 # Copyright (C) 1994 by Linus Torvalds
 #
 
-KBUILD_DEFCONFIG := defconfig
-
 NM := $(NM) -B
 
 LDFLAGS_vmlinux	:= -static -N #-relax

arch/arm64/Makefile

···
   endif
 endif
 
-KBUILD_DEFCONFIG := defconfig
-
 # Check for binutils support for specific extensions
 lseinstr := $(call as-instr,.arch_extension lse,-DCONFIG_AS_LSE=1)
 

arch/csky/Makefile

···
 # SPDX-License-Identifier: GPL-2.0-only
 OBJCOPYFLAGS		:=-O binary
 GZFLAGS			:=-9
-KBUILD_DEFCONFIG	:= defconfig
 
 ifdef CONFIG_CPU_HAS_FPU
 FPUEXT = f

arch/nds32/Makefile

···
 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

arch/riscv/Makefile

···
 KBUILD_AFLAGS_MODULE += -fPIC
 KBUILD_CFLAGS_MODULE += -fPIC
 
-KBUILD_DEFCONFIG = defconfig
-
 export BITS
 ifeq ($(CONFIG_ARCH_RV64I),y)
 	BITS := 64

arch/s390/Makefile

···
 # Copyright (C) 1994 by Linus Torvalds
 #
 
-KBUILD_DEFCONFIG := defconfig
-
 LD_BFD		:= elf64-s390
 KBUILD_LDFLAGS	:= -m elf64_s390
 KBUILD_AFLAGS_MODULE += -fPIC

scripts/kconfig/Makefile

···
 Kconfig := Kconfig
 endif
 
+ifndef KBUILD_DEFCONFIG
+KBUILD_DEFCONFIG := defconfig
+endif
+
 ifeq ($(quiet),silent_)
 silent := -s
 endif