* Log the fan speed at boot
Record the fan RPM (and the maximum seen) during boot, so that it is
accessible using "sudo vclog -m".
See: https://github.com/raspberrypi/rpi-eeprom/issues/678
* Add current_supply to HAT+ support
Refactor the HAT library to make it more self-contained, and combine
the I2C address detection and the reading of the EEPROM contents.
Use it to allow the earlier boot stages to check for a current_supply
setting in the EEPROM of a normal (non-stackable) HAT+.
