Working with your Overlay FAQ
Basic terms and concepts
Use cros deploy
If yes, then just add the package name to the DEPEND (if build-time dependency) and/or RDEPEND (if run-time dependency) of your app's ebuild.
Add it as an RDEPEND to chromeos-base/chromeos-bsp/chromeos-bsp-<board>.ebuild in your overlay.
Example BSP that pulls in peerd and bash on the target.
CrOS uses the upstart system to manage boot and service startup. Simply add an upstart job for your service and install it into /etc/init
Example upstart script: link
system-services, unless your job in the critical boot path (e.g. network driver setup), in which case boot-services is appropriate.
The following additional resources go into more detail: