image: don't override opkg list directory in per-device rootfs mode

image: don't override opkg list directory in per-device rootfs mode

opkg's -l option is always interpreted relative to the installation root.
This leads to very weird paths inside the rootfs (containing the whole path
to the LEDE tree on the build machine) and causes the subsequent deletion
of the list directory to fail (cluttering the resulting images).

Instead, use the default list directory and remove its contents in
prepare_rootfs.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>