Fix FPGA board deployment and move to chipflow-lib

[robtaylor]

deploying to FPGA is currently broken (minimal/design/steps/board.py)

It should be generalised and moved to chipflow-lib.

[robtaylor]

@gatecat has some work done here already

Everyone's mapping of peripherals onto an FPGA board is going to be different - you can't really know or force someone to put SPI on a specific header, for example. There's some common stuff we can factor out but that's the best we can get, it's just the way things are.

The plan is to have a datafile describing given boards, and a user config for pin assignment to headers etc.