MFC r263052, r263124, r263265, r263267...  Enhance loaderdev env var.

Enhance the mechanism that lets you configure the ubldr boot device by
setting the u-boot environment variable loaderdev=.  It used to accept only
'disk' or 'net'.  Now it allows specification of unit, slice, and partition
as well.  In addition to the generic 'disk' it also accepts specific
storage device types such as 'mmc' or 'sata'.

If there isn't a loaderdev env var, the historical behavior is maintained.
It will use the first storage device it finds, or a network device if
no working storage device exists.

99% of the work on this was done by Patrick Kelsey, but I made some
changes, so if anything goes wrong, blame me.

(Indeed, the 3 followup commits fixed things I got wrong on the first.)