I created a simple build script, which can build NetBSD/evbarm for Olinuxino-imx233 from scratch on Linux system.
Basically, you need to do following:
hg clone https://bitbucket.org/tkoskine/build-netbsd-imx233 cd build-netbsd-imx233 sh -x ./build-imx233.sh
After a while, you have kernel, base system sets, boot partition image (containing the kernel), and rootfs image (containing the sets) built.
However, at the moment, rootfs image is not ready to use. File owners are wrong and the system in general is unconfigured (no /etc/fstab, no /etc/rc.conf settings, ...).
Also, you need to figure out how to create correct partitions to the sd card. While in theory, it is doable from Linux, this is easiest to do from an existing NetBSD system.
Example binaries are available at files.tkoskine.me.