nios2: enable "make defconfig"

Running "make defconfig" currently fails for nios2:

    $ make ARCH=nios2 defconfig
    scripts/kconfig/conf --defconfig Kconfig
    ***
    *** Can't find default configuration "arch/nios2/defconfig"!
    ***
    make[1]: *** [defconfig] Error 1
    make: *** [defconfig] Error 2

Add a definition for KBUILD_DEFCONFIG to the nios2's main Makefile to enable this make target.

---
 arch/nios2/Makefile | 2 ++
 1 file changed, 2 insertions(+)

index e142c9ee51fa..2328f82ba2a8 100644
@@ -14,6 +14,8 @@
 # Nios2 port by Wind River Systems Inc trough:
 #   [email protected] and [email protected]
 
+KBUILD_DEFCONFIG := 3c120_defconfig
+
 UTS_SYSNAME = Linux
 
 export MMU

1.9.3

Applied.

Thanks.