Unify USB-related makefile var and C def as CIRCUITPY_USB. Always define it as 0 or 1, same as all other settings. USB_AVAILABLE was conditionally defined in supervisor.mk, but never actually used to #ifdef USB-related code. Loosely related to #4546
This can be enforced by pre-commit, but correct it separately to make it easier to review.
This adds support for Litex, along with support for the Fomu FPGA board. Signed-off-by: Sean Cross <sean@xobs.io>