Resetting doesn't clear loaded ROM image

[robinsonb5]

Once an image is loaded using romload or similar, the only way to clear it and return to MSX DOS is currently to power-cycle the board or re-upload the core

[robinsonb5]

The reset procedure already has provision for clearing the RAM-based ROM image but only uses it at power-on reset. It should be therefore be fairly straightforward to add a hard reset.