Das U-Boot (or just “U-Boot” for short) is an Open Source Firmware project for PowerPC, ARM, MIPS, x86 and Blackfin processors (to name a few!). The main U-Boot project is hosted on denx's website.
The source and documentation on our site is specific to the Blackfin processor. We try to keep things up to date, and periodically submit patches back to the mainline project. If you have questions about the Blackfin port of U-Boot, you can ask on our U-Boot forums.
Make sure you use the source code from the Blackfin U-Boot project rather than from upstream.
For some basics about U-Boot, please check: