Add include guard to `mbed_config.cmake`

[rwalton-arm]

Description

Some users may want to include mbed_config.cmake in their application CMakeLists.txt and modify some of the preset variables. This is currently impossible: the variables defined in mbed_config.cmake will be overwritten when the file is included again in the mbed-os root CMakeLists.txt.

We can use CMake's include_guard feature to force CMake to skip processing of the file if it has already been processed previously in the build.

Azure Pipelines have removed Python 3.6 from the macOS-latest runners. Python3.6 is nearing end of life so we can remove testing for it.

https://www.python.org/dev/peps/pep-0494/#lifespan

Test Coverage

• [x] This change is covered by existing or additional automated tests.
• [ ] Manual testing has been performed (and evidence provided) as automated testing was not feasible.
• [ ] Additional tests are not required for this change (e.g. documentation update).

[codecov[bot]]

Codecov Report

Merging #321 (fd9d186) into master (2a421e7) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #321   +/-   ##
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  Coverage   97.08%   97.08%           
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  Files          92       92           
  Lines        2779     2779           
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  Hits         2698     2698           
  Misses         81       81