Use a better Makefile and support the NexysVideo board as well

Summary of the updates:

A better Makefile to generate the Vivado project file automatically.
Support the NexysVideo Board.
Raise the core clock frequency from 30MHz to 60MHz (need some optimizations in Vivdao).
Have the rocket-chip and riscv-pk sub modules forked into CNRV to have better control.
Specify the versions (commit) of the riscv-gnu-toolchain repo.

cnrv / fpga-rocket-chip