Add cocotb verilator flow_tool option

[isomoye]

This PR adds verilator as a cocotb flow_tool simulation option by adding the cocotb "extend an existing build flow" options in sim.py. It also adds a switch to the verilator makefile call to use "Vtop.mk" instead of "V 'cocotb-module'.mk".

A cocotb simulation running on verilator can then be run using just the following fusesoc simulation configuration

  flow: sim
  flow_options:
    tool : verilator
    cocotb_module : test_file
    timescale: 1ns/1ns
  filesets : [rtl,cocotb]
  toplevel : [toplevel]```

[olofk]

I wouild prefer evaluating cocotb-config --share directly like we do with cocotb-config --libdir. That makes the script more portable if we want to run it on a different machine than it was created on.

[isomoye]

fixed by changing the share directory evaluation to a direct call in tools/verilator.py rather than a system call. see commit aa7d8c3.

[olofk]

Many thanks. A very welcome addition and I'm hoping to find some time to try it myself. I just had a smaller comment above. Happy to merge if you could fix that.

[olofk]

Thanks. Code looks fine now, but regression tests are failing and there seems to be two reasons for that.

1. Formatting needs to be redone. This is typically done with a pre-commit hook automatically, so just run pre-commit install from the repo root and any changed files will be reformatted correctly when you make a commit
2. The verilator regression tests fail because there is no flow_options key in the EDAM. The easiest way to get around this is probably to replace if self.edam["flow_options"].get("cocotb_module"): with something like if self.edam.get("flow_options", {}).get("cocotb_module"):, but I'm also thinking more philosophically if the tools are supposed to be looking at flow options at all. The other way to do this would be to add a cocotb_module tool option for Verilator and then define let the sim flow define the value of that option. Long-term we probably want to do something like that, but we can keep it simple for now and just make sure not to crash if flow_options is missing

[isomoye]

I fixed the format issue by following your suggestions. I agree with your assessment of using flow_options in the tool specific flow and I'm open to working on a permanent fix soon. I tested the verilator.py changes without flow_options and it looks like your suggested change fixed the crashing issue.

[olofk]

There was still some formatting issues in sim.py, but I took the liberty to fix that locally. Squashed and pushed! Thank you for your contributions!