Atari Keyboard keys that do not exist on a PS/2 keyboard

Atari - PS/2
HELP - DRUCK
UNDO - PAUSE/U
CLR-HOME - POS1

7812,5 bit/sec 8N1

Missing/TODO:

* Refactor the source code - clean up unused functions.
* Add licence headers
* Replace uart.c/.h with the one used in the bootloader.