I2C driver implementation (Master mode) for the SAMR21

Implementation of the I2C driver (Master mode) for the SAMR21 including:

- periph_conf.h: I2C configuration added
- i2c.c: I2C_0, implementation of all functions but slave mode

I do some "return;" when I detect an error which is not that good. Which solution do I have?
Please put the `else if` into a new line.