unicore32: rename unicore32_defconfig to defconfig

Since the initial support of unicore32, it has always had a single
defconfig. Rename it to 'defconfig', which is now the standard name
when arch has just a single defconfig file.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/unicore32/Makefile

@@ -41,8 +41,7 @@ libs-y			+= arch/unicore32/lib/
 
 boot			:= arch/unicore32/boot
 
-# Default defconfig and target when executing plain make
-KBUILD_DEFCONFIG	:= $(ARCH)_defconfig
+# Default target when executing plain make
 KBUILD_IMAGE		:= $(boot)/zImage
 
 all:	zImage