[INCOMPATIBLE CHANGE] Implement new deployment model

[INCOMPATIBLE CHANGE] Implement new deployment model

See https://wiki.gnome.org/OSTree/DeploymentModel2

This is a major rework of the on-disk filesystem layout, and the boot
process.  OSTree now explicitly supports upgrading kernels, and these
upgrades are also atomic.

The core concept of the new model is the "deployment list", which is
an ordered list of bootable operating system trees.  The deployment
list is reflected in the bootloader configuration; which has a kernel
argument that tells the initramfs (dracut) which operating system root
to use.

Invidiual notable changes that come along with this:

1) Operating systems should now come with their etc in usr/etc; OSTree
   will perform a 3-way merge at deployment time, and place etc in
   the actual root.  This avoids the need for a bind mount, and is
   just a lot cleaner.
2) OSTree no longer bind mounts /root, /home, and /tmp.  It is expected
   that the the OS/ has these as symbolic links into /var.

At the moment, OSTree only supports managing syslinux; other
bootloader backends will follow.