Modified I2C api to work with NRF's dynamic pin asignment.

Dependents:   Seed_Barometer_Sensor_custom_I2C_lib

Note that the only changes are in the constructor. I tried extending the original I2C class, but could not find away to avoid the i2c_init from being called there.

I2CX.cpp/shortlog@9d9fc94594e3: not found in manifest