Add I2C library and example for CC2538 

Add one header file and one implementation file to use the I2C peripheral of the CC2538 platform.
Add a line in the CC2538 makefile to include the I2C library for compilation.
Also add an example to use the I2C library with two different sensors (TCN75 from Microchip and SHT21 from Texas Instruments).

This adaptation of I2C library was made by Mehdi Migault for the 4MOD Technology company, as part of an internship with the IT department of the "Institut Universitaire et Technologique" in La Rochelle.
Parts of the sources come from Contiki and Texas Instruments.

It include an example for the I2C library that work with two different I2C sensors, the TCN75 from Microchip and the SHT21 from Texas Instruments. It measure temperature and send the value over the serial port.
I think it's probably prudent to leave it in, in case someone blindly passes in a non boolean value.