Documentation of "make fpga" for 32 bit

[nathpranay]

@zarubaf Current documentation says " Generating a Bitstream To generate the FPGA bitstream (and memory configuration) yourself for the Genesys II board run: make fpga " With reorg and changes in riscv_pkg for XLEN, " make fpga " does not work for 32 bit. Just "make fpga" make 64 bit CPU.

However "make fpga variant=32" may help generating Bitstream ( .mcs/.bit) for 32 bit CPU. Documentation changes may help a lot. ( Thanks @JeanRochCoulon for helping with suggession)

[zarubaf]

Yes agree 💯! Can you or somebody else put the changes forward for the README? Please base it on the release/0.4.3 branch.