sysbuild: mcuboot: Clarify monotonic counter usage

Clarifies that monotonic hardware counter will only be used on
the first image and disallow usage of this feature with QSPI XIP
split image updates as it is not compatible

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

sysbuild/Kconfig.mcuboot

@@ -16,10 +16,15 @@ menuconfig MCUBOOT_HARDWARE_DOWNGRADE_PREVENTION
 	bool "Downgrade prevention using hardware security counters"
 	depends on (SOC_NRF5340_CPUAPP || SOC_SERIES_NRF91X)
 	depends on !SECURE_BOOT_APPCORE
+	depends on !QSPI_XIP_SPLIT_IMAGE
 	help
 	  This option can be enabled by the application and will ensure that the
 	  MCUBOOT_HW_DOWNGRADE_PREVENTION Kconfig option is enabled in the MCUboot image.
 
+	  Note that this can only be used on first image, it will not be applied to the second
+	  image (network core updates) on nRF5340 which will use software downgrade protection
+	  on the network core CPU instead.
+
 if MCUBOOT_HARDWARE_DOWNGRADE_PREVENTION
 
 config MCUBOOT_HW_DOWNGRADE_PREVENTION_COUNTER_SLOTS