wic: rootfs: fix rootfs path reporting

wic: rootfs: fix rootfs path reporting

wic gets rootfs paths from partition object property
'rootfs_dir' and shows them in final report.

rootfs plugin sets this property to the temporary path,
which causes temporary paths appearing in the report.

Changed the code to prevent storing temporary rootfs path
in part.rootfs_dir. This should fix the report.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>