CMake: Update readme for greentea test build procedure

[rajkan01]

Summary of changes

• No longer need to create mbed_os.lib and soft link to mbed-os as new --mbed-os-path command-line removes such dependency and creates mbed_build.cmake when passing mbed-os path with that command-line argument.

Impact of changes

None.

Migration actions required

None.

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

[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

@0xc0170 @hugueskamba

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[ciarmcom]

@rajkan01, thank you for your changes.
@hugueskamba @0xc0170 @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

It's time to add the testing to CI, isn't it?

[rajkan01]

It's time to add the testing to CI, isn't it?

No, I requested one more change to Rob to add a set(MBED_PATH xyz_path) config into generated mbed_config.cmake whenever command line --mbed-os-path receives the mbed-os path which helps to avoid specify set(MBED_PATH) in every greentea test CMakeLists.txt file

[0xc0170]

CI started