E250 environmental monitoring

Hi all,

I've just committed more changes to current to extend the environmental
monitoring on the E250.  The fan speed is controlled automatically based
on the CPU temperature [*] and it's possible to show the status of some
components using `envstat` and show and alter the LED's using sysctl.
For example (with some values omitted for brevity):

  # envstat
                     Current  CritMax  WarnMax  WarnMin  CritMin  Unit
  [ecadc0]
           CPUFAN:       164        0        0        0        0  none
  [pcf8574io1]
     psu0_present:      TRUE
  [pcf8574io2]
    disk0_present:      TRUE
    disk1_present:      TRUE
  [pcf8574io3]
       key_normal:      TRUE

  # sysctl -a -e | grep hw.led
  hw.led.fault=0
  hw.led.activity=0
  # sysctl -w hw.led.activity=1

I've also added the additional details for the v240 (we already had details
for the v210).

Regards,

Julian

PS.  It should be possible to do similar for the E450, based on our previous
monitoring code, but I don't have access to one of those.

[*] In practice, the fan speed on my E250 hardly varies because the CPU
temperatures only vary a little (the fan speed is based on CPU temps).