The default SPI device configuration allows to define up to two SPI devices `SPI_DEV(0)` and `SPI_DEV(1)`. `SPI_DEV(0)` is always defined if the SPI peripheral is enabled by the module `periph_spi`. The second SPI device `SPI_DEV(1)` is only defined if `SPI_DEV_1_USED` is defined by the board. This allows to use the default configuration with one or two SPI devices depending on whether other peripherals are enabled that would collide with the SPI devices.
Furthermore, the CS signal in the SPI configuration is given by a define that can be overriden with another pin if
the default CS signal is connected to an unused hardware.
