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.