MCUBoot has been able to handle encrypted images for upgrade for a while. Add support for this feature to TF-M.
Includes removing the forked imgtool and using MCUBoot's version. Commits are set up so that one can choose between the version from the MCUBoot repository, or the one in the Python Package Index.

Commits:

• https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/4563
• https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/4564
• https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/4565