nRF54H20: MCUBoot can be tricked to executing unauthenticated code

When using the Direct XIP update strategy, the main application image starts other cores (i.e. radio core), based on the currently active slot without additional verification. The MCUboot in the bare (upstream) configuration assumes that if there is at least a single slot for each image available, the system is bootable and continues the boot process. This may lead to a situation when MCUboot picks different slot for different images (i.e. (a) for the main application and (b) for the radio image), boots the main application (from slot (a)) that afterwards starts the radio image by providing an address of the unauthenticated slot ((a) instead of (b)).