s390: move arch/s390/defconfig to arch/s390/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>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Makefile

@@ -10,6 +10,8 @@
 # Copyright (C) 1994 by Linus Torvalds
 #
 
+KBUILD_DEFCONFIG := defconfig
+
 LD_BFD		:= elf64-s390
 KBUILD_LDFLAGS	:= -m elf64_s390
 KBUILD_AFLAGS_MODULE += -fPIC