qemu_v8: ease SMP testing through QEMU_SMP

As for qemu.mk, environment can set the number of cores to be emulated
using QEMU_SMP. Defaults to 1 core.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>