Wafer thin Bazel layer on top of ORFS

[oharboe]

No question, just sharing to see if there are any likeminded that are interested in doing this.

If this is a common use-case, I was hoping to share & learn.

With the do-floorplan/place/route/finish/generate-abstract support in ORFS Makefile where the dependency checking is separated from the execution of those stages, I've successfully built a wafer thin Bazel layer on top of ORFS.

Using Bazel on top of ORFS gives me some nice advantages:

• Artifacts stored in Google Cloud. Some macros have .lef, .lib and .gds files that run into the 100s of megabytes and take a long time to build
• Distributed builds (I have not set up this yet, but I don't see a reason why it shouldn't work), which is nice if you are working from a laptop or a machine that doesn't have dozens to hundreds of cores.
• Dependencies can be expressed in more detail. For instance, I don't want to re-run synthesis, just because I'm modifying parameters for place/route.

I added do-gds support in #1543

The reason I'm not using https://github.com/hdl/bazel_rules_hdl, is that I want to have an environment that is as close to ORFS as possible so as to make it easier to report issues and get the most recent OpenROAD and ORFS updates immediately.

Perhaps later I would use https://github.com/hdl/bazel_rules_hdl.... but for now the priority is to work as closely as possible with ORFS/OpenROAD while having the Bazel advantage.

[oharboe]

I've put together an example of how to create a wafer thin Bazel layer on top of ORFS and how to use this from a repository that is not a fork of ORFS: #1550