contiki/dev/bme280

Contiki implementation for Bosch Sensortec BME280 sensor. 
BME280 is compact and fast and includes temp, RH, and pressure. 
Chip uses I2C or SPI. This implementation and follow Contiki 
device API and I2C.

For better performance and less chip warm up and less I2C transactions
burst read is recommended. Here all T/RH/P is read in one single 
I2C read and kept the measurements have the same time. 

The burst read is stored in struct bme280_mea which accessible from 
the Contiki. Also note that the full chip resolution is available. 
The variables are overscaled to give the app full controlover resolution.

See bme280.h

The pressure can be calculated with 32 or 64. The define BME280_64BIT
controls this. Typically in your project-conf.h
 
Implemented according to datasheet Rev 1.1.

Limitations:
Implementation implements Weather Mode which uses forced one-shot 
mode no oversampling nor filters and reads all T/RH/P in one read.