The default I2C device configuration allows to define up to two I2C devices `I2C_DEV(0)` and `I2C_DEV(1)`. `I2C_DEV(0)` is always defined if the I2C peripheral is enabled by the module `periph_spi`. The second I2C device `I2C_DEV(1)` is only defined if `I2C_DEV_1_USED` is defined by the board. This allows to use the default configuration with one or two I2C devices depending on whether other peripherals are enabled that would collide with the I2C devices.
