use rsync to copy data dir contents
use rsync to copy data dir contents

Changelog: Fix MENDER_DATA_PART_DIR sometimes copying some files
multiple times.

Based on a patch by David Silver <dsilver@upthere.com>

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender