unicore32: add KBUILD_DEFCONFIG with unicore32_defconfig (old debug_defconfig)

Rename debug_defconfig to unicore32_defconfig, which is a minimal config for
PKUnity-v3 (130nm) SoC board.
Also, add KBUILD_DEFCONFIG to use 'make defconfig'.

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>