I2C bus system (avr) + sensors SHT21, TLS2561, BMP085, BMA180

Signed-off-by: Kay Förster <red.mail2@gmail.com>
Include I2C sensors to the Contiki System

one project example: project/sn-sensoransteuerung
tested with avr-atmega128rfa1 platform

Signed-off-by: Kay Förster <red.mail2@gmail.com>