Das U-Boot (the universal bootloader), is arguably the richest, most flexible, and most actively developed FOSS bootloader available. It's released under the GNU GPL and maintained at http://www.denx.de/wiki/U-Boot/. Being a bootloader, uboot is not part of OpenWrt.

However being distributed under the GPLv2, and being quite well maintained, you are free to play around with it. Like OpenWrt is build (cross compiled) with OpenWrt Buildroot, uboot can be build with help of the Embedded Linux Development Kit. It is quite well documented: ELDK-4 / ELDK-5.

1. retrieve source code, patch support for you hardware if not already contained in that mainline version
2. use a toolchain of your choice to build

→http://www.denx.de/wiki/view/DULG/UBootDoesntRunAfterUpgradingMyCompiler

→ uboot.config
→ flashlog

If somebody writes a patch for the bootloader implementation of a particular device, you will find links to this on the wiki-page for that device. Yet we additionally accumulate all the patches wrote for a particular bootloader on his own page. Hopefully you can get a better comprehension of the functionality of the bootloader by having a look at them:

• For the Dockstar: https://github.com/doozan/uBoot by Jeff Doozan

• ftp://ftp.denx.de/pub/u-boot/ Obtain latest version
• http://www.denx.de/wiki/U-Boot/Documentation
  • Das U-Boot Presentation A short presentation for a quick overview
  • Das U-Boot online manual The Manual
  • FAQ
• Blackfin
• Porting U-Boot for a new Blackfin device Blackfin specific, but you can milk this for information
• U-Boot as coreboot payload just for fun