add setenv pass

[nakengelhardt]

What are the reasons/motivation for this change?

File lists (a verific feature) will often make use of environment variables in file paths, e.g. this example from the CV32E40P core. This provides a way to set these variables from within a yosys script.

Explain how this is achieved.

Add a setenv command that simply passes its arguments to the setenv() POSIX function.

If applicable, please suggest to reviewers how they can test the change.

There's a testcase included that can be used with verific.

In the absence of verific, it's possible to use this to set any other environment variable that yosys checks (although there aren't too many - you can change HOME or PATH and see what mayhem you can cause...) It would also be possible to use TCL to check that the value was updated correctly.

Unlike the TCL implementation, this is not thread-safe, but neither is most of the rest of yosys.

TODO:

• add the right guards for windows. @mmicko, could you have a look at that?
• handle quoted strings