Add pvmfw partition to target files

The pvmfw is included in an `m` build but not in the distribution files.
Apply the same treatment as the dtbo partition to copy the pvmfw
partition to dist/ since, similar to the dtbo image, the pvmfw image is
generally provided as a prebuilt image.

Test: make dist
Bug: 174457787
Change-Id: I6f42517ba42db92e90048d1236d7255ccbd73f73