Create an array for BTN0_PIN etc

This is kept more for reference than to actually merge -- while it does work, it is missing crucial information (not only on LEDs but even on BTNs) on what the pin's semantics are (most buttons are push-to-ground, but not all).

This would have been an easy way to get by at least some gpio_t for demo purposes.

The better way is likely to just look into gpio_params (even if it's not used by SAUL); then the API can even be by name or index at build time.