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.
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