Building netboot artifacts should not require setting boot modes

The netboot build mode depends on _make_boot_on_iso9660 and thus on ${bootmodes[@]} and _make_bootmode_* to get its artifacts.

This should to changed since netboot does not need any of the boot loaders or their files. Instead _build_buildmode_netboot could be changed to not depend on $isofs_dir and reimplement _make_boot_on_iso9660 using another directory to store the artifacts before copying them to $out_dir.