Add mbed OS version macros #3207
                
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Allow compile-time tests on the version of mbed-os to cope with
e.g. API changes across versions.
To distinguish between mbed OS 2 and mbed OS 5, we use the
MBED_CONF_RTOS_PRESENT macro.
Note: mbed OS 2 versioning is 2.0.MBED_LIBRARY_VERSION
Related PRs
This is #3096 rebased against master