.. _ug_bootloader_config:

Customizing the bootloader
##########################

.. contents::
   :local:
   :depth: 2

By default, to set Kconfig options, the |NSIB| and MCUboot bootloaders use Kconfig project configuration files, usually called :file:`prj.conf`, located in their source directories:

* |NSIB| - :file:`nrf/samples/bootloader`
* MCUboot - :file:`bootloader/mcuboot/boot/zephyr`

However, there are other ways to customize your application using Kconfig options:

* Using custom project configurations - For permanent options.
* Using Kconfig fragments - For temporary options.

Using custom project configurations
***********************************

You can use custom project configuration options for the associated image, specifying them at build time using :ref:`ug_multi_image_variables`, either temporarily until you clean the build pristinely or permanently.

For example, you can temporarily assign custom project configurations for both the bootloaders and a sample application as follows:

.. code-block:: console

   west build -b nrf52840dk/nrf52840 zephyr/samples/hello_world -- \
   -Db0_CONF_FILE=prj_immutable.conf \
   -Dmcuboot_CONF_FILE=prj_upgradable.conf \
   -DCONF_FILE=prj_app.conf

In the example above, :file:`prj_app.conf` includes :kconfig:option:`CONFIG_SECURE_BOOT` and :kconfig:option:`CONFIG_BOOTLOADER_MCUBOOT` to enable the immutable and upgradable bootloaders by default.
The configuration applied during the command execution is taken into account until you clean the build pristinely.

Alternatively, you can follow :ref:`ug_multi_image_permanent_changes` and store configuration options for child images in separate files in the application source directory.
For example, in the |NCS| applications and samples that use different :ref:`build types for configuration <gs_modifying_build_types>`, the :file:`child_image` folder in the application source directory is often used to apply :ref:`permanent configuration changes <configuration_permanent_change>`.

You can use custom project configuration files in combination with temporary configuration options associated with a single build, set using either the command line or Kconfig fragments.

Assigning Kconfig fragments
***************************

You can use Kconfig fragments specific to bootloaders to set temporary configuration options for the associated image, specifying them at build time using :ref:`ug_multi_image_variables`.

For example, you can assign the :file:`my-custom-fragment.conf` fragment to the |NSIB|, which uses the child name of ``b0``, as follows:

.. code-block:: console

   west build -b nrf52840dk/nrf52840 zephyr/samples/hello_world -- \
   -DCONFIG_SECURE_BOOT=y \
   -DCONFIG_BOOTLOADER_MCUBOOT=y \
   -Db0_EXTRA_CONF_FILE=my-custom-fragment.conf

In the same way, you can replace ``b0`` with ``mcuboot`` to apply the :file:`my-custom-fragment.conf` fragment to the MCUboot image:

.. code-block:: console

   west build -b nrf52840dk/nrf52840 zephyr/samples/hello_world -- \
   -DCONFIG_SECURE_BOOT=y \
   -DCONFIG_BOOTLOADER_MCUBOOT=y \
   -Dmcuboot_EXTRA_CONF_FILE=my-custom-fragment.conf

You can use this method to apply Kconfig fragments to any child image in the build, as well as to set any Kconfig option that can be set from the command line.

See :ref:`ug_multi_image_variables` for more information about customizing images using this method.

Customizing partitions
**********************

With the Partition Manager, you can further customize it if a dynamic partition map has been set.
For more information, see the :ref:`Configuration <pm_configuration>` section of the :ref:`partition_manager` page.