Support for FRU removal in dbus-sensors/hwmontempsensor

[spinler]

Need a change so that the sensor is removed from D-Bus when a cable card is removed, otherwise it will be unrecognizable from a regular communications error.

See openbmc/dbus-sensors#11 for details. Opening here for tracking purposes.

Note that right now nothing would clear the inventory when a card is removed, so entity-manager would never send an InterfacesRemoved for hwmontempsensor to find. Another option is to put GPIO support into hwmontempsensor, though we'd have to be careful about hwmontempsensor trying to access a chip before VPD sets the access GPIO, which sets the I2C address for the TMP435/TMP275. Could then either get a read for or a read of another device, depending on what the defaults are.

[spinler]

We're not going to go this direction, instead hwmontempsensor will do all the work of knowing when power is on. Closing.