populate-volatile.sh: don't spawn background process at rootfs time

populate-volatile.sh: don't spawn background process at rootfs time

If we're building a read-only rootfs, we'll get the following error now
and then.

    tar: .: file changed as we read it

The root cause is that we spawn background process at rootfs time.
When the tar command is running, it's possible that files under rootfs
are changed by background processes, thus this error.

[YOCTO #4937]

(From OE-Core rev: 4bd419f95868d5b8707a45cee5f6c5c6a840a65b)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>