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 <pebolle@tiscali.nl>
Acked-by: Ley Foon Tan <lftan@altera.com>