CMake: Rename Mbed OS path variable for consistency

[hugueskamba]

Summary of changes

Rename the CMake variable expected from applications from MBED_ROOT to
MBED_PATH to be consistent with the naming used for the configuration file
(MBED_CONFIG_PATH) also expected from applications.

Impact of changes

All supported applications need to be updated to use the new name..

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

[] 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

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[ciarmcom]

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

[0xc0170]

As examples were updated, and we will run a test with other change, I'll merge this in to get #13856 green