arch: add KBUILD_DEFCONFIG to allow "make defconfig" ...

Currently, "make defconfig" is only supported for sandbox and x86.

I set KBUILD_DEFCONFIG to support "make defconfig" where I was able
to find a suitable one.

nios2 and openrisc have a single generic_defconfig, so it is definitely
good for KBUILD_DEFCONFIG.

For arm, mips, I assume qemu is a preferred choice over a real board.

blackfin has only one defconfig. Since it is inactive (it is gone in
Linux), so no more defconfig will come in.

I am not sure about ppc and riscv, so I did not touch them.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

nios2: Fix out of tree build ...

nios2 links the board specific nios_sopc.h to include/. The relative
pathes used do not work with an out of tree build. Use absolute pathes
instead.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

make make -s more silent ...

use $(kecho) instead of echo to not output messages with
make -s

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Disable emitting mul instructions by gcc ...

If we are using barebox on a NiosII/e variant, the
mul instruction is not supported and causes an exception.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add Nios2 arch support to barebox ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>