That's right - these files are inside something called Board Support Package (BSP), essentially sources for the OS image. Check out the BSP Build Guide in the Documentation section to get started on this.
EDIT: And by the way, there's documentation in the works right now, which will describe how to run the board with rootfs on the SD card, so if you have the Arduino expansion board or something like SparkFun's SD card Block, you'll be able to increase your rootfs without rebuilding the image. I think that's going to be really helpful for a lot of people. I'll post in this thread when I have more details to share.
