Customizing the bootloader

By default, to set Kconfig options, the nRF Secure Immutable Bootloader and MCUboot bootloaders use Kconfig project configuration files, usually called prj.conf, located in their source directories:

  • nRF Secure Immutable Bootloader - nrf/samples/bootloader

  • MCUboot - 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. These settings are temporary and will remain enabled until you clean the build pristinely.

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

west build -b nrf52840dk/nrf52840 zephyr/samples/hello_world -- \
-Db0_FILE_SUFFIX=immutable \
-Dmcuboot_FILE_SUFFIX=upgradable \
-Dapp_FILE_SUFFIX=app

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.

For example, you can assign the my-custom-fragment.conf fragment to the nRF Secure Immutable Bootloader, which uses the image name of b0, as follows:

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

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

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

You can use this method to apply Kconfig fragments to any image in the build, and set any Kconfig option that is available from the command line.

See Sysbuild (System build) 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 Configuration section of the Partition Manager page.