usbcdc: provide doxygen only macro for doxy builder

[0xc0170]

Summary of changes

There is functionality hidden behind this macro. As result, doxygen won't include these. It should be
enabled to cover this functionality as well.

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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@iriark01 @ARMmbed/mbed-os-core

[0xc0170]

The predefined contain only drivers but there is BLE and TFM. My question is what should be in predefined? As I believe we are missing there much more?

Wait a sec, there is also json file - I am confused...

[0xc0170]

Updated also json file

[ciarmcom]

@0xc0170, thank you for your changes.
@iriark01 @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Still failing, not fixed the issue: requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://os.mbed.com/docs/mbed-os/6.0.0-preview/mbed-os-api-doxy/class_u_s_b_c_d_c___e_c_m.html

[0xc0170]

I updated the branch - force pushed to provide another way to get doxygen . Before I go down the rabbit hole to touch doxygen_options, we should try to use || defined(DOXYGEN_ONLY) that is already being used.

[0xc0170]

tested locally by running doxygen. Without a fix file mbed-os/BUILD/html/class_u_s_b_c_d_c___e_c_m.html is not generated - no doxygen for this object. With this PR, it is generated as previously was (5.15 contains this class documented).

[0xc0170]

CI started

Please review, if we can merge this asap to master to fix building the docs for 6.0.0beta1

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

Needs a review and should be good to go in.