phosphor-hwmon funcitonal interface

[Patrick Venture]

Right now, if a sensor has an explicit hwmon fault interface, then we
have the notion of a status:

https://github.com/openbmc/phosphor-hwmon/blob/master/sensor.cpp#L205

However, I propose that if there isn't a fault file, we still add the
interface, and set to functional=true.  This would allow us to change
it to false in the following condition:

https://github.com/openbmc/phosphor-hwmon/blob/master/hwmonio.cpp#L127

So basically, instead of returning -rc on failure (optionally), it
could optionally set functional to false, and phosphor-host-ipmid
would need to know to check this additional interface.

Thoughts?  (IIRC< I've mentioned this idea before, but now i'm setting
up some cycles to hammer it out, as I continue to implement unit-tests
in phosphor-hwmon)

Patrick