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.

Signed-off-by: Paul Bolle <[e-mail]>
Acked-by: Ley Foon Tan <[e-mail]>

arch/nios2/Makefile

···
 # Nios2 port by Wind River Systems Inc trough:
 #   [e-mail] and [e-mail]
 
+KBUILD_DEFCONFIG := 3c120_defconfig
+
 UTS_SYSNAME = Linux
 
 export MMU