Add rom dir support for os.rom, basic.rom, amsdos.rom

[erikarn]

Add support for the ROM directory, so the base ROMs can be added in. This allows for modified base ROMs, eg for testing out patched firmware ROMs.

This only applies for the non-plus models. The plus models use cartridges and the setup path is a little different.

Whilst here, flip on the drive config file handling, it works fine now.