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: NRandy 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: NYoshinori Sato <ysato@users.sourceforge.jp>

arch/h8300/Makefile

@@ -8,6 +8,8 @@
 # (C) Copyright 2002-2015 Yoshinori Sato <ysato@users.sourceforge.jp>
 #
 
+KBUILD_DEFCONFIG := edosk2674_defconfig
+
 cflags-$(CONFIG_CPU_H8300H)	:= -mh
 aflags-$(CONFIG_CPU_H8300H)	:= -mh -Wa,--mach=h8300h
 ldflags-$(CONFIG_CPU_H8300H)	:= -mh8300helf_linux