Always set the 'next-server' options when there is a TFTP subboss.
Always set the 'next-server' options when there is a TFTP subboss.

Previously, we were not doing this if there was a DHCP subboss (which,
in theory, should make the next-server line unnecessary since the client
should ignore us entirely). I have a vague recollection of some PXE caching
the wrong next-server value and this seemed to help at PRObE.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome