Running prepare_rootfs on TARGET_DIR deletes the opkg state when
CONFIG_CLEAN_IPKG is enabled, making the per-device rootfs package install
fail.

To avoid this, create a copy of the TARGET_DIR before prepare_rootfs is run
and use this as basis for per-device rootfs generation.

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