If it's not first boot, /tmp has already been symlinked to /var/volatile/tmp.
But the udev service starts before populate-volatile.sh starts. This leads to
a dead link at /tmp. As a result, trying to create any file under /tmp will
fail.
If a USB is plugged in before the populate-volatile.sh script starts, the
/tmp/.automount-$name file will not be created correctly. As a result, when
the USB is unplugged, the /media/$name directory is not removed.
So we create /var/volatile/tmp directory in the udev script to avoid this dead
link problem.
