* Modify Doxygen grouping of `drivers` Public/Internal APIs
* Correct classification of `mbed_events.h`
* Amend name of Doxygen group containing Device Key API
* Classify `CallChain.h` as public API and relocate file
* Remove Doxygen group from `equeue_platform.h` as it has no Doxygen compliant documentation
* Move USB target specific code back to `usb/device/targets`
* Change Doxygen groups structure, splitting first by Public/Internal
This commit also does the following:
* groups the documentation of related API
* moves `events/internal/equeue.h` to `events/equeue.h`
* merges `events/source/README.md` to `events/README.md`
The contents of the usb directory were moved to appropriate locations and the usb directory removed.
* Public USB headers moved under drivers/
* Internal USB headers moved under drivers/internal/
* USB Source code moved under drivers/source/usb/
* Moved usb/device/hal/ under hal/usb/
* Moved usb/device/USBPhy/ under hal/usb/
* Merged usb/device/targets/ into targets/
* Separated public and private USB API documentation under Doxygen groups drivers-public-api and drivers-internal-api.
Hugues Kamba
George Psimenos