First download and install imx31_ads_20071008-rel5b-ltib.iso.
Download and extract u-boot-1.3.3.tar.bz2.
Place toolchain on PATH:
$ export PATH="$PATH:/opt/freescale/usr/local/gcc-4.1.2-glibc-2.5-nptl-3/arm-none-linux-gnueabi/bin/"
Export the CROSS_COMPILE name to U-Boot
$ export CROSS_COMPILE=arm-none-linux-gnueabi-
Enter in the u-boot-1.3.3 directory.
Create configuration to i.MX31ADS board:
$ make mx31ads_config
Compile U-Boot to selected board:
$ make
If compilation goes fine you will see the u-boot.bin on current directory.