doc: hardware: porting: remove old hwmv1 section

   ├── Kconfig.plank

   ├── Kconfig.defconfig

   └── plank_<new-qualifiers>.yaml

Board extensions (Old hardware model)

*************************************

.. note::

  This extension mechanism is intended for boards in old hardware description

  format. For boards described in new hardware model format, use the extension

  feature described in :ref:`extend-board`.

Boards already supported by Zephyr can be extended by downstream users, such as

``example-application`` or vendor SDKs. In some situations, certain hardware

description or :ref:`choices <devicetree-chosen-nodes>` can not be added in the

upstream Zephyr repository, but they can be in a downstream project, where

custom bindings or driver classes can also be created. This feature may also be

useful in development phases, when the board skeleton lives upstream, but other

features are developed in a downstream module.

Board extensions are board fragments that can be present in an out-of-tree board

root folder, under ``${BOARD_ROOT}/boards/extensions``. Here is an example

structure of an extension for the ``plank`` board and its revisions:

.. code-block:: none

   boards/extensions/plank

   ├── plank.conf                # optional

   ├── plank_<revision>.conf     # optional

   ├── plank.overlay             # optional

   └── plank_<revision>.overlay  # optional

A board extension directory must follow the naming structure of the original

board it extends. It may contain Kconfig fragments and/or devicetree overlays.

Extensions are, by default, automatically loaded and applied on top of board

files, before anything else. There is no guarantee on which order extensions are

applied, in case multiple exist. This feature can be disabled by passing

``-DBOARD_EXTENSIONS=OFF`` when building.

Note that board extensions need to follow the

:ref:`same guidelines <porting-general-recommendations>` as regular boards. For

example, it is wrong to enable extra peripherals or subsystems in a board

extension.

.. warning::

   Board extensions are not allowed in any module referenced in Zephyr's

   ``west.yml`` manifest file. Any board changes are required to be submitted to

   the main Zephyr repository.