initscripts: populate-volatile.sh: run create_file synchronously

The behavior of running create_file in the background was introduced in
d44816be ("initscripts: Populate volatile from existing file")
without further explanation. Besides its questionable benefit, this
causes actual issues:

- Not all create_file processes may have finished by the time the
  initscript exits (or when it moves /etc/volatile.cache.build !)
- By making the order of commands nondeterminstic, it could hide
  dependency issues where it was attempted to create files before their
  containing directories

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>