Add support of ostree to initramfs

Add new overlay for 'ostree' feature.
Allow to boot ostree-based OS deployment according to boot
parameter 'ostree=....'. Additional parameter 'os=...' may be used
to indicate OS name ("apertis" by default).

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

Differential Revision: https://phabricator.apertis.org/D7576

overlays/ostree/usr/share/initramfs-tools/scripts/init-bottom/ostree

+#!/bin/sh -e
+
+# Original version of 'switchroot.sh' from ostree upstream:
+# https://github.com/ostreedev/ostree/blob/master/src/switchroot/switchroot.sh
+
+sysroot=$rootmnt
+os=${os:=apertis}
+
+## 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