trenz icezero added, note read TUTORIALS/IceZero.md for spi flash tools

[njheb]

Proposal to add Trenz IceZero hx8k board. I didn't really know where best to add the raspberry pi spi flash tools so have kept them out in a separate tree. If you would be interested in adopting this board into your tree then I can make edits to follow your preference.

Right now Ice4Pi also gets a couple of tweaks including running the spi flash in a less basic mode, but I'm not certain it will work on older Ice4Pi boards. I've left it in as I expect I'll be required to make edits anyway before this pull request is acceptable.