Travis CI migration to GitHub Actions

[saheerb]

Summary of changes

Mbed OS is moving GitHub Actions from TravisCI. This PR takes care of the migration of mbed-os-5.15 branch.

Impact of changes

Migration actions required

Disable mbed-os-5.15 branch in TravisCI settings.

Documentation

None

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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

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

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Reviewers

@jamesbeyond

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[ciarmcom]

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

[jamesbeyond]

LGTM

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 10 of 10 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

@donatieng as @andypowers is away, can you review (approval is required for 5.15 PRs) ?

[donatieng]

Thanks @saheerb, LGTM!

[0xc0170]

Mbed OS is moving GitHub Actions from TravisCI. This PR takes care of the migration of mbed-os-5.15 branch.

Worth adding here:

• psa travis job was not ported - we do not regenerate libraries (no changes to psa) - same as on master
• littlefs was not ported - we got integrations tests. No need to run littlefs own tests (they run in littlefs upstream repository)