OpenBMC uses Yocto/Open-Embedded for a build system, which supports an out-of-tree build. It is recommended that you create an empty directory somewhere to hold the build. This directory will get big.
On Ubuntu 14.04 the following packages are required to build the default target
sudo apt-get install -y git build-essential libsdl1.2-dev texinfo gawk chrpath diffstat
To start a build:
cd <builddir>
. <repodir>/openbmc-env
bitbake obmc-phosphor-image