add the possibility to customize the delimiter of the ATHandler in AT…

[Tobi15]

…CellularDevice class

Summary of changes

Modify the constructor of the ATCellularDevice to permit to customize ATHandler delimiter

Impact of changes

Migration actions required

Documentation

Basically there is no specific change for all applications. But after this change, it would be possible to modify the ATHandler delimiter. By default the value is "\r".

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[x] 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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[ciarmcom]

@Tobi15, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Styling check failed (easy to fix), unittests failed (connectivity/cellular/tests/UNITTESTS/doubles/AT_CellularDevice_stub.cpp file requires an update)

[0xc0170]

Basic Checks / style-check (pull_request)

It is failing due to styling errors, please review

Pull request has been modified.

[0xc0170]

CI started

[mbed-ci]

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