use #S/sdctl to find a list of sd controllers to probe
for nvram or 9fat/plan9.nvr.  this allows for nvram
on any sd device without them being present in nvtab.
this allows nframe from, e.g. sdE1 (ahci) and sde0 (aoe)
and in the future, hopefully sda0 (combined mode sata/sas
controllers like the marvell orion).