update tools for making recovery images

We no longer render animations as a base image with a
possibly-partially-transparent overlay drawn over it, so delete the
make-overlay.py tool.  Now we represent them as series of images that
are interlaced by row (with a special text chunk in the PNG file
specifying the number of frames) so add the interlace-frames.py tool
to make those.

Change-Id: I866db269107a21351c3df3b4683f233f72234334