Compile U-Boot:

$ cd $WD
$ git clone git://git.toradex.com/u-boot-toradex.git -o toradex -b 2014.04-toradex u-boot
$ cd u-boot
$ make colibri_imx6s256m_config
$ make u-boot.imx
$ export PATH=$(pwd)/tools:$PATH
$ mkimage -V
mkimage version 2014.04