It is possible to temporarily suspend USB and safely preserve its
configuration. This is needed to allow a device to enter deep
sleep as a USBDevice instance prevents deep sleep. USB operation can be
suspended with `deinit` and restored with `connect`.
* 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`
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.