[TF-M] [RFC] Variable naming refinement in manifest tool

Hi all,

I made a patchhttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/7817 to refine the variable naming in manifest tooling, specifically the "manifest.manifest" in templates. I'm changing it to "partition.manifest" which should be more accurate and easy understanding. Broadcasting here for anyone has out-of-tree templates. And any comments are welcome.

Best Regards, Kevin