default the host in fetch urls to be ROOTSERVER if it is blank

default the host in fetch urls to be ROOTSERVER if it is blank

Reason: this would mean I don't have to customise live.cfg for each
server I deploy it on (because the tftp server also serves the root
image).

So:

     append fetch=tftp:///path/file.ext

doesn't work now, but after the patch this would be
assumed since the host is blank:

     append fetch=tftp://${ROOTSERVER}/path/file.ext

Closes: #800498