overlay: Ignition tweaks to work with composefs

This is to enable ostree+composefs:
ostreedev/ostree#2867

When we care about the *physical* backing filesystem, we need to
look at /sysroot/sysroot (which in the real root is `/sysroot`)
because now `/sysroot` (aka `/` in the real root) is a composefs
(really an `overlayfs` with a transient loop-mounted erofs), which
is distinct from the physical root.

Co-authored-by: Colin Walters <walters@verbum.org>

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-boot-edit.sh

@@ -32,6 +32,7 @@ rm -vrf ${initramfs_firstboot_network_dir}
 root=$(karg root)
 if [ -z "${root}" ]; then
     rdcore rootmap /sysroot --boot-mount ${bootmnt}
+    echo "Prepared rootmap"
 fi
 
 # This does a few things:

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/coreos-check-rootfs-size

@@ -4,7 +4,8 @@ set -euo pipefail
 # See also ignition-ostree-check-rootfs-size.service
 # https://github.com/coreos/fedora-coreos-tracker/issues/586#issuecomment-777220000
 
-srcdev=$(findmnt -nvr -o SOURCE /sysroot | tail -n1)
+# /sysroot is the mounted deploy root, /sysroot/sysroot is the physical root filesystem
+srcdev=$(findmnt -nvr -o SOURCE /sysroot/sysroot | tail -n1)
 size=$(lsblk --nodeps --noheadings --bytes -o SIZE "${srcdev}")
 
 MINIMUM_GB=8