6809 CPU Card for RC2014

I finally got back to working on this board set up a bit more and fixing  a couple of annoying bugs. I've pushed a revised ROM for the board to fix the following

- Early boot wrongly accessed and read every I/O port. This happened to be harmless for most cases

- The correct checksum is now computed for the ROM so the boot properly displays ROM... Passed

It also adds some initial code to support booting another OS off disk instead of Cubix. I need to test and play with it but it ought to be sufficient to let people port things like Flex and NitrOS9 to the board (and of course Fuzix in my case).