U-Boot, or to give its full name, Das U-Boot, began life as an open source bootloader for embedded PowerPC boards. Then, it was ported to ARM-based boards and later to other architectures, including MIPS and SH. It is hosted and maintained by Denx Software Engineering. There is plenty of information available, and a good place to start is http://www.denx.de/wiki/U-Boot. There is also a mailing list at u-boot@lists.denx.de.
U-Boot
Building U-Boot
Begin by getting the source code. As with most projects, the recommended way is to clone the .git archive and check out the tag you intend to use, which, in this case, is the version that was current at the time of writing:
$ git clone git://git.denx.de/u-boot.git
$ cd u-boot
$ git checkout...