doc: samples: Clarify on how to enable USB support in certain samples

It's now also needed to set the DTC_OVERLAY_FILE variable in order to
enable USB support in certain samples.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

doc/nrf/ug_thread_tools.rst

@@ -87,7 +87,7 @@ To program the nRF device with the RCP application, complete the following steps
 
          .. code-block:: console
 
-            west build -p always -b nrf52840dongle_nrf52840 nrf/samples/openthread/coprocessor/ -- -DOVERLAY_CONFIG="overlay-rcp.conf ../cli/overlay-thread_1_2.conf overlay-usb.conf"
+            west build -p always -b nrf52840dongle_nrf52840 nrf/samples/openthread/coprocessor/ -- -DOVERLAY_CONFIG="overlay-rcp.conf ../cli/overlay-thread_1_2.conf overlay-usb.conf" -DDTC_OVERLAY_FILE="usb.overlay"
 
       .. tab:: nRF52840 Development Kit (UART transport)
 

samples/bluetooth/peripheral_uart/README.rst

@@ -135,7 +135,7 @@ Activating sample extensions
 To activate the optional extensions supported by this sample, modify :makevar:`OVERLAY_CONFIG` in the following manner:
 
 * For the minimal build variant, set :file:`prj_minimal.conf`.
-* For the USB CDC ACM extension, set :file:`prj_cdc.conf`.
+* For the USB CDC ACM extension, set :file:`prj_cdc.conf`. Additionally, you need to set :makevar:`DTC_OVERLAY_FILE` to :file:`usb.overlay`.
 
 See :ref:`cmake_options` for instructions on how to add this option.
 For more information about using configuration overlay files, see :ref:`zephyr:important-build-vars` in the Zephyr documentation.

samples/openthread/cli/README.rst

@@ -120,7 +120,7 @@ To activate the optional extensions supported by this sample, modify :makevar:`O
 * For the experimental Thread 1.2 variant, set :file:`overlay-thread_1_2.conf`.
 * For the minimal single protocol variant, set :file:`overlay-minimal_singleprotocol.conf`.
 * For the minimal multiprotocol variant, set :file:`overlay-minimal_multiprotocol.conf`.
-* For USB transport support, set :file:`overlay-usb.conf`.
+* For USB transport support, set :file:`overlay-usb.conf`. Additionally, you need to set :makevar:`DTC_OVERLAY_FILE` to :file:`usb.overlay`.
 
 See :ref:`cmake_options` for instructions on how to add this option.
 For more information about using configuration overlay files, see :ref:`zephyr:important-build-vars` in the Zephyr documentation.

samples/openthread/coprocessor/README.rst

@@ -126,7 +126,7 @@ The following configuration files are available:
 * :file:`overlay-minimal_rcp.conf` - Enables a minimal configuration that reduces the code size and RAM usage.
   This file enables the RCP architecture with basic functionality and optimizes stacks and buffer sizes.
   For more information, see :ref:`app_memory`.
-* :file:`overlay-usb.conf` - Enables emulating a serial port over USB for Spinel communication with the host.
+* :file:`overlay-usb.conf` - Enables emulating a serial port over USB for Spinel communication with the host. Additionally, you need to set :makevar:`DTC_OVERLAY_FILE` to :file:`usb.overlay`.
 
 Building and running
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