So, to put the image on the SD card that showed up on my system as /dev/da2:
xz -d < FreeBSD-11.0-RC3-arm-armv6-BANANAPI.img.xz | dd of=/dev/da2 bs=1m
This puts the u-boot image (a SPL header which sets up the system to run u-boot coupled with u-boot) onto the disk. You can also uncompress the image you download and insert it there before pushing it out to the SD card. Since I also have a Banana PI I wanted to create an SD card for, I just changed it after it was on the image.dd if=/usr/local/share/u-boot/u-boot-pcduino/u-boot-sunxi-with-spl.bin of=/dev/da2 bs=1k seek=8 conv=notrunc,sync
Unfortunately, every board needs a slightly different u-boot image. I have a number of changes I'm testing to help unify the u-boot ports. One of them adds metadata to each u-boot port that a generic imaging tool can use to make things better in the future.