integral-image-ndvm-pkg: Get files on the FILESPATH by making a symlink from THISDIR to INTEGRAL_EXPORT_DIR.

This seems preferrable as opposed to including the full path in
FILES_${PN} which causes files copied to ${WORKDIR} to have a weird
path. do_install was fixed up to account for this as well. There's
some cleanup in here too.
integral-image-ndvm-pkg: Move manifest check to a new task and do it in a shell.