Move Cortex specific RTX behind TARGET_CORTEX ...

Update directory structure to include RTX for only cortex targets, and
for all cortex targets. This patch accomplishes this by moving mbed-os
specific RTX files and RTX itself into rtos/TARGET_CORTEX along with
removing TARGET_CORTEX_M from the RTX5 directory.

The old directory structure:

rtos/rtx5/<mbed-os specific RTX files>
rtos/rtx5/TARGET_CORTEX_M/*
rtos/rtx4/*
rtos/<mbed-os specific RTX files>

Is re-arranged to:

rtos/TARGET_CORTEX/rtx5/*
rtos/TARGET_CORTEX/rtx4/*
rtos/TARGET_CORTEX/<mbed-os specific rtx files>

This both encapsulates RTX code more cleanly and makes it easier to
experiment with non-cortex cmsis-os2 backends, such as a posix
based cmsis-os2 backend.

Note - A potentially better name for the CORTEX_M directory would be
something like FEATURE_RTX5 since this directory only contains RTX5
related files. This cannot be done because there is not an easy way
to turn this feature on, since it cannot be done from mbed_lib.json.

Turn on doxygen for DEVICE_* features ...

Enable doxygen for all device features. Also fix the warnings that
showed up once this was enabled.

Remove RTOS from Doxygen exception list (with the exception of CMSIS/RTX directores)

Removed Doxygen errors from Event.h ...

- Removed Makefile from branch
- Moved a0..a4/c0..c4 parameter documentation from top level overloaded functions to each function to appease Doxygen.
- Removed @see Event::Event from overloaded functions as the link did not work and the argument parameter documentation was moved down to each function
- TODO: Cleanup EventQueue.h

Enable doxygen for nsapi

Remove more warnings from doxygen config

Remove warnings from doxyfile_options

Update doxyfile_options that has settings used by developer.mbed.org ...

Should be kept in sync with doxygen_options.json which is used for
building the doxygen which is used by docs.mbed.com It is intended
only generate doxygen for specific directories which present the
public APIs for mbed OS