Added button pins array for iterating

If you wanna change the button order of the arrays (eg. to match konami code) we should do it soonish for consistency.

I'm not certain but I'm guessing that A1,A2, etc are #defined as ints describing their pin numbers. I can't guarantee that we will get pins 0 and 1 working easily for select and start, as they are RX and TX, but we should be able to disable RX and TX since we don't need it. Worst case is that we break out some of the pins on the back of the board (there's a bunch more including 3 touch ones, and we also have not used A5 yet).