integral-image-ndvm-pkg: Make missing manifest files a warning.

Previously these were uncaught exceptions. Since the anonymoust
python function is run when the recipe is parsed having this
uncaught or treated as an error makes things ... difficult. Changing
the way the export class functions (as in changing these file names)
causes the parse step in the pkg recipe to fail horribly.

Making these just a warning at least get us past the parse phase.
integral-image-ndvm: Make the NDVM rootfs read-only.