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/includes/program_sample.txt

@@ -8,4 +8,8 @@ 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 using the MCUboot board variants with KMU (default), ``west flash --erase`` or ``west flash --recover`` must be used during first programming of a board to program the KMU with the keys.
+
 For more details, see `How to flash an application`_ in |VSC| extension documentation.