That said - you should be able to use "wic" tool within your Yocto environment to build a complete image you could dd over according to what you specified. You might have only partition 1 as fat but the rootfs e.g. in an ext3 partition
Note tools like dd will wear and tear your SD or USB device prematurely, see:
https://www.google.com/search?q=olpc%20how%20to%20damage%20flash
