System ROM

The system ROM implements a Commodore-ish kernel built from the ground up. It will support a subset of API calls from the Commander X16 ported to RISCV along with additional calls I have found useful in my own programs.

At present, some basic drawing routines exist which is enough to get BASIC working.

Whether BASIC ends up in ROM is a question I will have to tackle later. For now, BASIC runs on the system as a program. My plan is to have a simple program run in ROM if the SD card isn't detected in the system.