doxygen: Set cwd to the Doxyfile folder
doxygen: Set cwd to the Doxyfile folder

The doxygen tool had previously been modified to always run from the
root of the build directory.  This was done as a workaround to allow
the OUTPUT_PATH in the doxyfile to contain a relative path.  The tool
then had to be modified to fix up any relative paths in the Doxyfile
to be relative to the root of the build directory.  This is problematic
because there are many DoxyFile parameters that can be relative paths.

This patch fixes up the logic to execute doxygen in the same directory
where the DoxyFile lives.  This allows all relative paths in the
Doxyfile to work so they no longer need fixed up.  It then updates the
logic for OUTPUT_DIRECTORY to handle relative paths correctly.

Signed-off-by: Thomas Nagy <d86e025e60d31fa2452cf443d0fed051ea678233@gmail.com>

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