initramfs-framework: mount /run and move to rootfs before switch_root

initramfs-framework: mount /run and move to rootfs before switch_root

Mount /run as tmpfs during early init and include it in the set of
mounts moved to $ROOTFS_DIR prior to exec switch_root.

Having /run available early lets initramfs modules stamp state that can
later influence systemd service jobs, since systemd will reuse the mount
point instead of creating a new one during boot.

This is particularly useful with ostree, as it uses /run/ostree-booted
as way to describe that the rootfs comes from an ostree deployment.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>