Support both BME280 pinouts

The board currently only supports the 4-pin I²C variant of the BME280. There is a 6-pin variant that supports I²C and SPI that is arguably more popular, e.g.:

@qwindelzorf noted that we can and should support both variants. The first 4 pins are identical, and so long as we hard-wire the bus select pin to select I²C, then either chip can be used with the same firmware.