sh: select KBUILD_DEFCONFIG depending on ARCH

You can not select KBUILD_DEFCONFIG depending on any CONFIG option
because include/config/auto.conf is not included when building config
targets.  So, CONFIG_SUPERH32 is never set during the configuration,
then cayman_defconfig is always chosen.

This commit provides a sensible way to choose shx3/cayman_defconfig.

arch/sh/Kconfig sets either SUPERH32 or SUPERH64 depending on ARCH
environment, like follows:

  config SUPERH32
          def_bool ARCH = "sh"

          ...

  config SUPERH64
          def_bool ARCH = "sh64"

It should make sense to choose the default defconfig by ARCH,
like arch/sparc/Makefile.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/sh/Makefile

@@ -15,6 +15,12 @@ ifneq ($(SUBARCH),$(ARCH))
   endif
 endif
 
+ifeq ($(ARCH),sh)
+KBUILD_DEFCONFIG	:= shx3_defconfig
+else
+KBUILD_DEFCONFIG	:= cayman_defconfig
+endif
+
 isa-y					:= any
 isa-$(CONFIG_SH_DSP)			:= sh
 isa-$(CONFIG_CPU_SH2)			:= sh2
@@ -105,14 +111,12 @@ ifdef CONFIG_SUPERH32
 UTS_MACHINE		:= sh
 BITS			:= 32
 LDFLAGS_vmlinux		+= -e _stext
-KBUILD_DEFCONFIG	:= shx3_defconfig
 else
 UTS_MACHINE		:= sh64
 BITS			:= 64
 LDFLAGS_vmlinux		+= --defsym phys_stext=_stext-$(CONFIG_PAGE_OFFSET) \
 			   --defsym phys_stext_shmedia=phys_stext+1 \
 			   -e phys_stext_shmedia
-KBUILD_DEFCONFIG	:= cayman_defconfig
 endif
 
 ifdef CONFIG_CPU_LITTLE_ENDIAN