Ignition tweaks to work with composefs

I'm currently working on composefs use in ostree:
 ostreedev/ostree#2640

I'm running into issues with ignition when testing it.
Several places look at /sysroot and try to find the backing
filesystem for it. This works because /sysroot is typically a
bind-mount of some deploy directory in the real sysroot.
However, in the composefs case, /sysroot is a composefs mount
and doesn't have a real backing block device.

This change switches these cases to look at /sysroot/sysroot instead.
This is the actual real sysroot after ostree-prepare-root did its
work (and before pivot-rooting into it).

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

@@ -31,7 +31,7 @@ rm -vrf ${initramfs_firstboot_network_dir}
 # append rootmap kargs to the BLS configs.
 root=$(karg root)
 if [ -z "${root}" ]; then
-    rdcore rootmap /sysroot --boot-mount ${bootmnt}
+    rdcore rootmap /sysroot/sysroot --boot-mount ${bootmnt}
 fi
 
 # This does a few things:
@@ -42,4 +42,4 @@ fi
 # 3. it create a .root_uuid stamp file on the bootfs or fails if one exists
 # 4. it adds GRUB bootuuid.cfg dropins so that GRUB selects the boot filesystem
 #    by UUID
-rdcore bind-boot /sysroot ${bootmnt}
+rdcore bind-boot /sysroot/sysroot ${bootmnt}

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

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