Added support for cores outside of riscv-formal repository

[stevehoover]

Hey, Clifford! I've incorporated riscv-formal runs into my WARP-V CI testing. These changes will help me clean up the process a bit. They let me run with riscv-formal as a submodule in the warp-v repo. All they do is make 'basedir' in genchecks configurable. Be aware that I'm new to Python.

I'm not sure how you would prefer to handle pull requests. I don't see a regression test, per se, but I ran pico and Vex with and without the change and got the same errors both ways :).

Sorry about my timing. I'm sure you are busy preparing for ORConf. I hope it goes well!

[cliffordwolf]

This will break existing cores. The "run from within riscv-formal/cores/<core>" assumption is bogus. There are different tools being run in different directories (for example most checks are run in riscv-formal/cores/<core>/checks) that all inherit this basedir string. It must be an absolute path.

[stevehoover]

Finally getting back to this. It looks like I was allowing the --basedir argument to be a relative path, in which case I append os.getcwd() after arg processing. Thus prepending os.getcwd() on line 29 isn't necessary as it will happen below. If you'd prefer to keep line 29 as it was, I can put another pull request together.