Define Wake build rules in this repository

[Richard Xia]

/cc @albertchen-sifive, @erikdanie

I think it makes sense to define the Wake build rules in this repository, as opposed to in https://github.com/sifive/api-generator-sifive/tree/1538a86cf354c0979a6a70ebd7dfbf90eb802d67/fpga, since it'll keep that in sync with this repo. It'll also allow us to keep on top of the different FPGA board types and any special behavior for each type of board.

Wake 0.18 will make it easier to make this transition, since it has a module system, so we could actually keep both sets of rules active and let the consumers choose which set to import from.