misc/yosys-config.in: don't use the non-portable '==' operator with test(1)

[thorpej]

What are the reasons/motivation for this change?
The '==' operator is non-standard and does not work with all versions of test(1) or sh(1).

Explain how this is achieved.
The fix is simple: use the standard '=' operator instead.

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

Resolves issue #4665.

[widlarizer]

The file is shebanged with #/usr/bin/env bash and bash on different platforms should be consistent, I'm not really sure how does sh come into play here, but since = and == is strictly equivalent in bash, this PR is good to go