Uboot source code: Where can I find it?

Title. I want to do some experimenting with booting off of USB or SD without needing an OS on the Yún's internal flash. My intentions with this are an attempt to get Debian (or some other Linux distribution booting, if at all possible), and possibly also make myself a MIPS development environment for porting Haiku. I did search through the OpenWrt repositories, but what I find are several folders with an Uboot-something name. If this is the source, I'm not sure at all which single folder the Yún uses for its own Uboot, or wether it uses all, or none. I need help here, guys.

That's the folder

Uboot sources are on github as well: they are in that tar.bz2 you see listed

Thanks a ton. Hopefully, I'll have a MIPS workstation out of this bugger soon enough.