The Yocto Project is an open source collaboration project that provides templates, tools and methods to help you create custom Linux-based systems for embedded products regardless of the hardware architecture
OpenBMC uses the Yocto tools to manage configuration and creation of BMC images.
There are two main use-cases for Yocto in OpenBMC:
The first is the easy case, and largely involves picking the system configuration to build before invoking bitbake. Examples for Palmetto and Barreleye are in the cheatsheet.
The second case can be helped with Yocto's devtool. After running . openbmc-env, a tool called devtool will be in your path, and can be applied in several ways.
If you have an existing source tree you'd like to integrate, running devtool modify -n ${PACKAGE} ${SRCTREE} first creates a new Yocto layer in your build directory where devtool stores recipe modifications. It then constructs a .bbappend for the the package recipe and uses the externalsource class to replace the download, fetch, and patch steps with no-ops. The result is that when you build the package, it will use the local source directory as is. Keep in mind that the package recipe may not perform a clean and depending on what you are doing, you may need to run ${PACKAGE} build system's clean command in ${SRCTREE} to clear any built objects. Also if you change the source, you may need to run bitbake -c cleansstate ${PACKAGE} to clear BitBake's caches.
Alternatively if you don't already have a local source tree but would still like to modify the package, invoking devtool modify ${PACKAGE} will handle the fetch, unpack and patch phases for you and drop a source tree into your default workspace location.
When you are all done, run devtool reset ${PACKAGE} to remove the .bbappend from the devtool Yocto layer.
Further information on devtool can be found in the Yocto Mega Manual.