Fixes for doc build (#270)

Document moving and rename. Fixes for latest xdoc.

doc/dfu/rst/xmos_dfu.rst

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+Using the DFU loader - macOS (via the XMOS DFU loader)
+======================================================
+
+The XMOS DFU loader is provided as source as part of the XMOS USB Audio software
+framework, see :ref:`usb_audiosec_building_xmos_dfu`.
+
+NOTE: Windows requires the installation of libusbK drivers on the DFU endpoint.
+We recommend using `Zadig <https://zadig.akeo.ie/>`_.
+
+Set up the image loader
+-----------------------
+
+#. Open a terminal
+#. Change directory to where the loader has been built
+#. Point DYLD_LIBRARY_PATH to libusb/OSX64:
+
+    ``export DYLD_LIBRARY_PATH=$PWD/libusb/OSX64:$DYLD_LIBRARY_PATH``
+
+Download new firmware
+---------------------
+
+To program the new firmware run the command:
+
+   ``./bin/xmosdfu XMOS_L2_AUDIO2_PID --download new_firmware.bin``
+
+Replace ``XMOS_L2_AUDIO2_PID`` with product ID of your target device. Invoke
+``xmosdfu`` with no arguments to get a list of all supported product IDs.
+
+Note that once this is done the device restarts. The original factory default
+application is still present but the device is now running the upgraded
+application firmware.
+
+Uploading existing firmware from the device
+-------------------------------------------
+
+You can retrieve a firmware image from the device, providing an upgrade image is
+present.
+
+Run the command:
+
+  ``./bin/xmosdfu XMOS_L2_AUDIO2_PID --upload currentfirmware.bin``
+
+The file ``currentfirmware.bin`` contains the latest upgrade image. This file is
+an exact copy of the data from the flash and can be downloaded to the device
+again to test.
+
+Reverting firmware to factory image
+-----------------------------------
+
+To revert the device back to its factory (i.e XFLASH) installed state from the
+new firmware, run the command:
+
+  ``./bin/xmosdfu XMOS_L2_AUDIO2_PID --revertfactory``
+
+The device will now be running, and only contain the factory firmware.