8.0 booter: chain-boot sd0 if present?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I'm not sure which list is most appropriate for this; this is my guess.
If I've got it wrong, just let me know where I should send it instead
and I'll move it there.

I'm working on a commercial turnkey product running under amd64/8.0.
They are looking for suitable machines (their current hardware is a bit
long in the tooth) and they've found a machine, but it's got a problem,
and I'm wondering how easy it would be to work around it in the NetBSD
booter.

The problem is field upgrades.  Their current hardware is set up so
that field upgrades consist of plugging in a USB thumbdrive and
rebooting.  The machine boots from the thumbdrive, the install happens,
the user removes the thumbdrive and reboots, and everything is good.

It appears to be impossible to configure the new machines to boot from
a newly-plugged-in thumbdrive in preference to the internal SSD without
BIOS interaction.  Requiring BIOS interaction for upgrades is not
suitable for their purposes; the machines normally do not have
keyboards, and requiring one for upgrades would be a significant
regression in functionality.

Obviously, one alternative is to simply consider such machines
unsuitable for their purposes.  But, by that definition, it seems to be
really hard to find suitable hardware.  In principle, they may be able
to get machines with a tweaked BIOS, but, if past experience is any
indication, finding a vendor that understands what they want accurately
enough to get a suitable BIOS set up is likely to be difficult.  So I
was looking for other alternatives.

It occurred to me that in principle it should be possible to work
around this in the booter, that the booter on the internal SSD could
check for the thumbdrive and, if found, chain-boot it.  But I don't
know how easy that would be to do with the 8.0 booter.

Thoughts?  Alternatives?

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