** the [http://www.chromium.org/chromium-os/developer-guide chromium os developer guide] might also be of interest, among other things it shows that google do split the build into a package building part and an image creation part.
* [https://wiki.sabayon.org/?title=En:Entropy entropy] is sabayons portage replacement, it focuses on binaries due to sabayon being a binary distribution
** their [https://github.com/Sabayon/build build system "Matter" ] might be of interest (although I haven;t found any info), also it seems to automate large parts of tracking gentoo portage with its tinderbox subsystem** sabayon has <code>kernel-switcher</code> for updating kernels
** kernel ebuilds live [https://github.com/Sabayon/sabayon-distro/tree/master/sys-kernel/linux-sabayon here] and probably rely on the [https://github.com/Sabayon/sabayon-distro/blob/master/eclass/sabayon-kernel.eclass sabayon-kernel eclass].