build: fix verity generation

* Now build_verity_metadata.py takes an arg telling it what to do,
  in this case "build" is our intention, so update our custom runcmd
  array.

* Ensure we cast all args to Popen as strings or picky python gets
  angry that we pass an int (image_size) to Popen

Change-Id: I71c5e45e3155f470259f91f6f1a880e780aef369