A Chromium-based OS can take a long time to build. However, you can perform many optimizations to speed up this process. Currently, the Chromium OS build system is driven by bash scripts that don't detect what items have changed. If you blindly use these scripts, you will be overbuilding and wasting precious time.
This page goes through each of the bash(or python) scripts, explaining what they do and when you have to run them. Another source of information is the -h option for each script.
This script creates the local chroot environment using the a prebuilt SDK chroot and prebuilt packages. Use the --replace option to replace a previous chroot environment (re-running this command will not delete your previous chroot). Once it has created the local chroot environment, cros_sdk will also chroot into the SDK and set up basic enviroment. If you prefer to just build and not enter, please use the --download option. In terms of what you see, this is the development environment that all packages see when they are building. This script resides in chromite, but should be accessible in your PATH if you have recent enough depot_tools.
This script builds your Chromium source for use with Chromium OS (special flags and environment changes are necessary for this to happen correctly). It then packages Chromium into a ZIP file that is used in build_image.sh.
This script must be run outside of the chroot environment and needs to be re-run whenever major Chromium changes happen. Since building Chromium takes a long time, unless you are making changes to Chromium, avoid syncing and rebuilding whenever possible. This script is based around the build process of Chromium and so it is more intelligent and will only build what is necessary. You can also test the resulting executable without running it on a Chromium OS system.
This script does most of the building. It runs through all the directories in third_party and under platform and runs their make_pkg.sh script. make_pkg.sh builds the .deb file for that package to be installed into the final Chromium OS image.
If you only make changes to one package, you only need to call its make_pkg.sh (from within chroot of course) and copy/install that deb to your Chromium OS device. This will save you a lot of time and prevent from having to rebuild the image every time. You will need to modify this script if you add any packages yourself.
This scripts builds the kernel.
Builds the Chromium OS image. This script goes through all the packages in the ~/src/build/x86/local_packages and all the other packages listed in the prod text file in package_repo to create the Chromium OS image. The image is then placed under ~/src/build/images. From here you can convert the image to a VMWare image or install it to a USB drive.
