The only change from v1 is how the temperature is encoded. In v2, two
bytes at offset 4 and 5 represent a 16-bit signed int (in big-endian).
It contains the temperature in degrees Celsius * 100.
Fixes#29
In the BLE service data payload, one of the reserved bits is now used to
indicate whether or not the ambient lux value is encoded in the packet.
Bit 0 of the first byte is now the `has_lux` bit. If it is set, bytes
16-17 in the service data will contain the ambient lux. If `has_lux` is
not set, bytes 16-17 may not exist (for compatibility with older
firmwares) or may hold meaningless values.
This commit also introduces version-specific configuration in
prst_config.h. Photoresistor config, for example, is only meaninful for
version 1.1.x.
