ostree: create file `/run/ostree-booted` in initramfs

This file is required by recent libostree versions for several
operations like `ostree admin upgrade` to be sure we are booted in
ostree-based system. So just create this file in initramfs script
responsible for mounting ostree structure.

Check APERTIS-5762 for additional details.

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