arch/h8300: add a defconfig target

Make the "defconfig" target valid for arch/h8300.  Currently
"make ARCH=h8300 defconfig" produces:

*** Can't find default configuration "arch/h8300/defconfig"!
../scripts/kconfig/Makefile:87: recipe for target 'defconfig' failed

By adding a value for KBUILD_DEFCONFIG, "make ARCH=h8300 defconfig"
successfully produces a kernel .config file:

*** Default configuration is based on 'edosk2674_defconfig'

This is useful for Kconfig editing/testing.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: uclinux-h8-devel@lists.sourceforge.jp (moderated for non-subscribers)
Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>