Ux5/V2x5 envmon sensor

[Julian Coleman <jdc%coris.org.uk@localhost>]

Hi,

I've just added the driver for the env-monitor on U25/U45/V215/V245 to
-current.  On the U25/U45, the front panel temperature is now reported via
`envstat`.  On the V215/V245, temperatures and fan speeds are reported via
`envstat` and front panel LED's can be viewed/changed via `sysctl hw.led`.

A big thank you to Ken Wellsch for testing on his V215!

Regards,

Julian

PS.  The V445 might have a similar sensor, but I don't have access to one
to test.  The driver won't attach on unknown machines, so we won't report
anything there.

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