doc: nrf-bm: includes: program_sample: Add note about MCUboot KMU usage

Add note about MCUboot KMU usage and key provisioning to sample
programming template.

Signed-off-by: Eivind Jølsgard <[email protected]>

Author: eivindj-nordic

doc/nrf-bm/images/erase_and_flash_to_board.png

@@ -8,4 +8,15 @@ Follow these steps to program this sample:
 #. To flash the application you have built, select your build folder in the `Applications View`_.
    Then, click :guilabel:`Flash` in the `Actions View`_ after connecting the device to the terminal.
 
+.. note::
+
+  When programming a board for the first time using MCUboot board variants with KMU (default), make sure the KMU is programmed with the necessary keys.
+
+  * Using the command line: Execute ``west flash --erase`` or ``west flash --recover``.
+  * Using Visual Studio Code: In the Actions View, hover over the :guilabel:`Flash` option.
+    Click :guilabel:`Erase and Flash to Board`, which appears to the right under :guilabel:`More actions`.
+
+   .. figure:: /images/erase_and_flash_to_board.png
+      :alt: Erase and flash to board.
+
 For more details, see `How to flash an application`_ in |VSC| extension documentation.