build_rootfs.sh: use nproc for qemu SMP emulation

Building rootfs on host with less than 8 CPU cores will cause
the kernel inside qemu to hang with the following errors:

  watchdog: BUG: soft lockup - CPU#3 stuck for 983s! [swapper/3:0]

Using `nproc` to detect the number of host cores instead of
providing a fixed value for qemu fixes this problem.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I3a68a1db47c5ff657dd6e09b3f81e4b3413ec5e9