wic/plugins/rootfs: Fix permissions when splitting rootfs folders across partitions

This patches makes locating the file database containing the file and
folder usernames and permissions more reliable.
In addition to locating it relative to the partition directory, we also
try to locate it relative to the IMAGE_ROOTFS.

Prior to this patch, the database was not found when using
--rootfs-dir=${IMAGE_ROOTFS}/<x> in the WIC script, leading to erronous file
permissions and ownership.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>