doc: application: improve Kconfig setting documentation

For information on available kernel configuration options, including

inter-dependencies between options, see the :ref:`configuration`.

Application Configuration

=========================

Setting Application Configuration Values

========================================

As described above, an application can use :makevar:`CONF_FILE` to

override its default kernel configuration.

This section describes how to edit Zephyr configuration

(:file:`.conf`) files.

#. Navigate to the :file:`app`, and create the :file:`prj.conf` file. Enter:

- Add each configuration entry on a new line.

   .. code-block:: bash

- Enable a boolean option by setting its value to ``y``:

  .. code-block:: none

     CONFIG_SOME_BOOL=y

  To ensure that a boolean configuration option is not set, add a line

  like this instead (including the leading ``#`` symbol):

       $ vim prj.conf

  .. code-block:: none

   The application's default configuration is specified by

   ``CONF_FILE``, ``CONF_FILE`` defaults to :file:`prj.conf` unless

   configured otherwise in the application :file:`CMakeLists.txt`

   file.

     # CONFIG_SOME_BOOL is not set

#. Edit the file and add the appropriate configuration entries.

- You can set integer and string options as well, like this:

   a) Add each configuration entry on a new line.

  .. code-block:: none

   b) Begin each entry with ``CONFIG_``.

     CONFIG_SOME_INT=42

     CONFIG_SOME_STRING="the best value ever"

   c) Ensure that each entry contains no spaces

- Ensure that each entry setting an option contains no spaces

  (including on either side of the = sign).

   d) Use a # followed by a space to comment a line.

- Use a # followed by a space to comment a line:

  .. code-block:: none

     # This is a comment.

   The example below shows a comment line and a board

   configuration override in the :file:`prj.conf`.

The example below shows a comment line and an override setting

:option:`CONFIG_PRINTK` to ``y``:

.. code-block:: c

    # Enable printk for debugging

    CONFIG_PRINTK=y

#. Save and close the file.

.. _override_kernel_conf:

Overriding Default Configuration