toolchain: doc: Custom toolchain doc updated for other.h requirement

Zephyr will then include the toolchain cmake files located in the

:file:`TOOLCHAIN_ROOT` directory:

- :file:`cmake/toolchain/generic.cmake`: configures the toolchain for "generic"

  use, which mostly means running the C preprocessor on :ref:`devicetree

  input files <dt-input-files>`.

- :file:`cmake/toolchain/target.cmake`: configures the toolchain for "target"

  use, i.e. building Zephyr and your application's source code.

- :file:`cmake/toolchain/<toolchain name>/generic.cmake`: configures the

  toolchain for "generic" use, which mostly means running the C preprocessor

  on the generated

  :ref:`devicetree` file.

- :file:`cmake/toolchain/<toolchain name>/target.cmake`: configures the

  toolchain for "target" use, i.e. building Zephyr and your application's

  source code.

Here <toolchain name> is the same as the name provided in

:envvar:`ZEPHYR_TOOLCHAIN_VARIANT`

See the zephyr files :zephyr_file:`cmake/generic_toolchain.cmake` and

:zephyr_file:`cmake/target_toolchain.cmake` for more details on what your

:file:`generic.cmake` and :file:`target.cmake` files should contain.

settings in a file named :file:`my-toolchain.cmake`, you can then invoke cmake

as ``cmake -C my-toolchain.cmake ...`` to save typing.

Zephyr includes :file:`include/toolchain.h` which again includes a toolchain

specific header based on the compiler identifier, such as ``__llvm__`` or

``__GNUC__``.

Some custom compilers identify themselves as the compiler on which they are

based, for example ``llvm`` which then gets the :file:`toolchain/llvm.h` included.

This included file may though not be right for the custom toolchain. In order

to solve this, and thus to get the :file:`include/other.h` included instead,

add the set(TOOLCHAIN_USE_CUSTOM 1) cmake line to the generic.cmake and/or

target.cmake files located under

:file:`<TOOLCHAIN_ROOT>/cmake/toolchain/<toolchain name>/`.

When :makevar:`TOOLCHAIN_USE_CUSTOM` is set, the :file:`other.h` must be

available out-of-tree and it must include the correct header for the custom

toolchain.

A good location for the :file:`other.h` header file, would be a

directory under the directory specified in :envvar:`TOOLCHAIN_ROOT` as

:file:`include/toolchain`.

To get the toolchain header included in zephyr's build, the

:makevar:`USERINCLUDE` can be set to point to the include directory, as shown

here:

.. code-block:: console

   west build -- -DZEPHYR_TOOLCHAIN_VARIANT=... -DTOOLCHAIN_ROOT=... -DUSERINCLUDE=...

.. _cmake option:

   https://cmake.org/cmake/help/latest/manual/cmake.1.html#options