'make ARCH=riscv defconfig' ends up with endless recursion.
Users may get upset with it.

In Linux, every architecture does something for 'make defconfig'.
Set something to KBUILD_DEFCONFIG to make it work for riscv and ppc.

There is only one file in arch/riscv/configs/, so there is no choice
but erizo_generic_defconfig for riscv.

There are some choices for ppc, but I am not sure what is appropriate.
I just picked something.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>