initscripts: Populate volatile from existing file

In some cases, it may be useful to populate a volatile file from an
existing one, e.g. a file in a read-only rootfs that may be edited in a
read-write destination.

To provide this behavior, creation of volatile files has been updated to
copy a file which has been given in the <linksource> field. If set to
none, the current behavior is preserved.

Signed-off-by: David Vincent <freesilicon@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>