Add --dpi-hdr-only option

[veripoolbot]

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Author Name: Todd Strader (@toddstrader)
Original Redmine Issue: 1491 from https://www.veripool.org

Original Assignee: Todd Strader (@toddstrader)

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Towards #�. I have the feature coded, but am not sure about the best way to test this:
https://github.com/toddstrader/verilator-dev/tree/dpi-header

Ideally, I'd like to run something like t_dpi_import with and without --dpi-hdr-only (to two different output directories). Then I'd check to make sure that with the flag, the DPI header is the only thing that comes out and make sure it is the same as the file produced without the flag. I'm not sure if driver.pl will support something like this. I need to do some reading to figure that out, but if there are other suggestions for testing I'm all ears.

[veripoolbot]

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Original Redmine Comment
Author Name: Todd Strader (@toddstrader)
Original Date: 2019-08-23T22:31:15Z

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It looks like I can just call compile() twice with different flags. I should be able to make something happen when I can get back to this.

[veripoolbot]

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Original Redmine Comment
Author Name: Todd Strader (@toddstrader)
Original Date: 2019-08-24T12:12:03Z

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I added a test for the new option. I believe this is ready to land now:
https://github.com/toddstrader/verilator-dev/tree/dpi-header

[veripoolbot]

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Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2019-08-27T23:02:00Z

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I'm fine with your squashing and pushing this.

However in the larger picture (perhaps worth discussing in the other bug) I'd prefer we not need to run the generation of the dpi headers as a separate step.

[veripoolbot]

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Original Redmine Comment
Author Name: Todd Strader (@toddstrader)
Original Date: 2019-08-28T01:44:32Z

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Pushed since I've used a similar feature in ModelSim for purposes other than what we're discussing in #�.

[veripoolbot]

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Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2019-08-29T23:14:53Z

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In 4.018.