doc: mcuboot: mention DFU for output file usage

Added mention of DFU in line 24 about the output files.
NCSIDB-856.

Signed-off-by: Grzegorz Ferenc <[email protected]>

Author: greg-fer

doc/mcuboot/readme-ncs.rst

@@ -7,7 +7,7 @@ The |NCS| includes MCUboot-specific integration files located in the :file:`modu
 
 You can use MCUboot in the |NCS| in one of the following roles:
 
-* As an `Immutable first-stage bootloader`_ that can perform device firmware updates to the application.
+* As an `Immutable first-stage bootloader`_ that can perform device firmware updates (DFU) to the application.
   Enable :kconfig:option:`SB_CONFIG_BOOTLOADER_MCUBOOT` in sysbuild to use it in this role.
 * As an `Second-stage upgradable bootloader`_ that can perform device firmware updates to both itself and the application.
   Enable both :kconfig:option:`SB_CONFIG_BOOTLOADER_MCUBOOT` and :kconfig:option:`SB_CONFIG_SECURE_BOOT_APPCORE` in sysbuild to use it in this role.
@@ -21,7 +21,7 @@ See the following user guides for more information on adding, configuring, and t
 * `Customizing the bootloader`_
 * `Signature keys`_
 
-When you add MCUboot to your application build, the files that can be used for firmware over-the-air (FOTA) upgrades are automatically generated.
+When you add MCUboot to your application build, the |NCS| build system will automatically generate files that you can use for firmware upgrades (DFU or FOTA).
 See the `MCUboot output build files`_ page for a list of all these files.
 
 Using MCUboot in the direct-xip mode