Make several smaller changes in preparation of TF-M using the external MCUBoot repo entirely.
Commits:
https://review.trustedfirmware.org/c/trusted-firmware-m/+/3823
https://review.trustedfirmware.org/c/trusted-firmware-m/+/3825/4
https://review.trustedfirmware.org/c/trusted-firmware-m/+/3826/4