cross-busybox-cron.xml: use build triggers to build cross-busybox

- Left old CURL code in there for now, so it can be used as an example
cross-busybox.xml: remove old build directory

- Enabled artifacting of busybox after each build, but only save the
  latest artifact
- Don't exit the build before copying the busybox binary into
  $WORKSPACE for artifacting