System Initialization for CoreOS

CoreOS uses systemd for init and process management. This repo is divided into three collections of files:

configs: Customized daemon configuration files, usually installed into /etc but really the sky is the limit there.
scripts: Helper scripts for init and service startup. These are generally to be used as systemd oneshot services and installed into /usr/lib/coreos.
systemd: Unit files for mount points, our helper scripts, or other services that doen't install their own unit files. There is a top level coreos-startup.target which depends on everything that should be enabled by default on CoreOS. See Ordering for details.

The coreos-base/coreos-init ebuild handles the install process.

Important Steps

A few notes on things that must happen which are unique to Core OS.

resize state: Support easy VM growth by checking if there is unused space at the end of the disk and expanding the filesystem to use it.
mount /media/state: Anything that should persist across boots must be in here bind mounted or linked here including /home and /var.
initialize /media/state: The state partition is created during build but we need to be sure that it has the directories we expect before trying to use it.
bind mounts: Map things into /media/state or /run and so on as apropraite.
mount /usr/share/oem: Provides extra vendor add-ons.
generate ssh keys: The stock sshd units do not handle this so we need to.
setup dev/debug mode: Optionally bind dev_image to /usr/local and remount / as read-write.

Dependencies between the units can be not so obvious to a human and our boot involves an unusually long sequence of actions:

resize-state.service Runs scripts/resize_stateful_partition
local-fs-pre.target
media-state.service and usr-share-oem.mount Other things are likely being mounted around here too.
init-state.service Create directories needed for home, var, and var-run.
home.mount and var.mount Almost a normal looking system now!
var-run.mount Binds /var/run to /run to consolidate clutter.
local-fs.target Mounting complete!
systemd-tmpfiles-setup.service Recreates assorted files and directories based on tmpfiles.d configs.
sysinit.target Things are looking usable now! Most daemons will start about now.