- v2 openprom disk boot generates "/sbus/esp/sd@3,0" which we translated
into "/sbus@0,0/esp@0,0/sd@3,0"... this caused esp.c to reject
the bootpath because it was looking for sbus slot/offset 0,0.
changed the code to generate "/sbus0/esp0/sd@3,0" to fix this
panic.
- when generating fake boot paths, generate them with unit numbers
rather than slot/offsets (thus, /obio@0,0 becomes /obio0, as
does vmel and vmes).
- fixed up comments to reflect these changes
tested new code on: 4/280 (diskless), 4/360 (diskless), 4/360 ("xd" disk),
4/370 (scsi disk), ss1 w/v1 openprom (diskless),
ss1 w/v1 openprom (scsi disk), ss2 w/v2 openprom (diskless),
ss2 w/v2 openprom (scsi disk).
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