boot.mount: mount by UUID instead

This makes the mount more resistant to other filesystems labeled `boot`
that may get plugged in at any point.

Part of: coreos/fedora-coreos-tracker#976

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

@@ -46,6 +46,14 @@ if [ ! -f "/sysroot/${mntfile}" ]; then
     mpath=$(karg rd.multipath)
     if [ -n "${mpath}" ] && [ "${mpath}" != 0 ]; then
         bootdev=/dev/disk/by-label/dm-mpath-boot
+    else
+        eval $(blkid -o export "${bootdev}")
+        if [ -z "${UUID}" ]; then
+            # This should never happen
+            echo "Boot filesystem ${bootdev} has no UUID" >&2
+            exit 1
+        fi
+        bootdev="/dev/disk/by-uuid/${UUID}"
     fi
 
     devservice=$(systemd-escape -p "${bootdev}" --suffix=service)