switchroot: Document a bit more, add demo shell implementation

This could help others who want to integrate with other init
systems/initramfs.

Commit-message-by: Colin Walters <walters@verbum.org>

Closes: #784
Approved by: cgwalters

src/switchroot/switchroot.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+# This demonstration script is an implementation in shell
+# similar to ostree-prepare-root.c.  For a bit more information,
+# see adapting-existing.md.
+
+## the ostree boot parameter is avaialbe during the init
+env | grep ostree
+# ostree=/ostree/boot.1/.../.../0
+## bind mount the ostree deployment to prepare it for move
+mount --bind $sysroot$ostree $sysroot$ostree
+## bind mount read-only /usr
+mount --bind $sysroot$ostree/usr $sysroot$ostree/usr
+mount --bind -o remount,ro $sysroot$ostree/usr $sysroot$ostree/usr
+## bind mount the physical root
+mount --bind $sysroot $sysroot$ostree/sysroot
+## bind mount the var directory which is preserved between deployments
+mount --bind $sysroot/ostree/deploy/os/var $sysroot$ostree/var
+## make sure target directories are present within var
+cd $sysroot$ostree/var
+mkdir -p roothome mnt opt home
+cd -
+## move the deployment to the sysroot
+mount --move $sysroot$ostree $sysroot
+## after these the init system should start the switch root process