Enable any LiteX-supported board to be used.

CFU-Playground currently supports Arty 35T and Nexys Video boards. The difference between them is very small, but the current setup requires duplication of at least a couple of files in the soc/ directory (arty.mk, arty.py, nexys_video.mk, nexys_video.py). Instead, the board should be a parameter so that just one .mk and one .py file are needed.

Of course, not all LiteX-supported boards have the fabric resources to handle a "full" VexRiscv variant, or the memory to handle both the loaded executable and the necessary work space (e.g. for the tensor arena), so we may want to perform some up-front checks.