add an extra function to specify the I2C bus

The I2C bus appears as /dev/i2c-2 on the HB i2eX under ArchLinuxArm - I don't know if it can appear as /dev/i2c-0 on some other devices, so the original behaviour is maintained, with an additional function int wiringHBI2CSetupB(int devId, int busId) if you need a different bus number