system: Use SPIpad for input on AVR
game: Fix pause function

The pause routine should:
1. Wait 100 ms before attempting to read the pause key again, to avoid
debouncing.
2. Wait for the pause key to be released.
3. Wait another 100 ms before resuming, to allow the button to be
released.
system: Add delay function

On PC, it is a wrapper around SDL_Delay().
On AVR, it polls the get_ticks() function.