initscripts: let populate-volatile.sh create the /tmp link
Previously, the /tmp link (/tmp -> /var/tmp) was created by the
bootmisc.sh script. So in case of a read-only rootfs, this symlink
would not be created correctly.
The populate-volatile.sh script is intended to handle all directories
and files related to volatile storage, so we should let it create
the /tmp link.
In addition, because of the improments of populate-volatile.sh, the data
loss problem of bug#3404 is also resolved by this patch.
