OpenMote-CC2538 sensor drivers

Several changes on the OpenMote-CC2538 platform and general OpenWSN file organization and content:
- Modifed .gitignore file to avoid committing temporary files
- Moved the SPI and I2C interfaces to the board directory instead of the radio directory
- Changed the I2C interface to one that is more complete
- Added an I2C driver implementation for the OpenMote-CC2538
- Added drivers for the ADXL346, SHT21 and MAX44009 sensors
Currently the code compiles but has not been tested. However, the code is backported from another CC2538 implementation that is known to work, so the code should work as well. Also notice that the new I2C interface breaks compatibility with existing sensor code in the Z1 platform. Needs to be fixed if appropriate.