alpha: move arch/alpha/defconfig to arch/alpha/configs/defconfig

As of Linux 5.1, alpha and s390 are the last architectures that
have defconfig in arch/*/ instead of arch/*/configs/.

  $ find arch -name defconfig | sort
  arch/alpha/defconfig
  arch/arm64/configs/defconfig
  arch/csky/configs/defconfig
  arch/nds32/configs/defconfig
  arch/riscv/configs/defconfig
  arch/s390/defconfig

The arch/$(ARCH)/defconfig is the hard-coded default in Kconfig,
and I want to deprecate it after evacuating the remaining defconfig
into the standard location, arch/*/configs/.

Define KBUILD_DEFCONFIG like other architectures, and move defconfig
into the configs/ subdirectory.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Paul Walmsley <paul@pwsan.com>

arch/alpha/Makefile

@@ -8,6 +8,8 @@
 # Copyright (C) 1994 by Linus Torvalds
 #
 
+KBUILD_DEFCONFIG := defconfig
+
 NM := $(NM) -B
 
 LDFLAGS_vmlinux	:= -static -N #-relax