coreos-copy-firstboot-network.sh: Place stamp file if copied

We would like to mount `/boot` read-only in the real root,
so remove the current 15-coreos-firstboot-network.conf since
it would not work once `/boot` is mounted ro. Drop a stamp
file instead so that `coreos-boot-edit.service` would notice
and perform the clean up later in the initramfs.

xref #659

overlay.d/05core/usr/lib/dracut/modules.d/15coreos-network/coreos-copy-firstboot-network.sh

@@ -26,10 +26,9 @@ if [ -n "$(ls -A ${initramfs_firstboot_network_dir} 2>/dev/null)" ]; then
     echo "info: copying files from ${initramfs_firstboot_network_dir} to ${initramfs_network_dir}"
     mkdir -p ${initramfs_network_dir}
     cp -v ${initramfs_firstboot_network_dir}/* ${initramfs_network_dir}/
-    # If we make it to the realroot (successfully ran ignition) then
-    # clean up the files in the firstboot network dir
-    echo "R ${realroot_firstboot_network_dir} - - - - -" > \
-        /run/tmpfiles.d/15-coreos-firstboot-network.conf
+    # Drop stamp file in /run to indicate that there are firstboot networking
+    # configuration files in /boot that should be cleaned up after Ignition.
+    touch /run/coreos-copy-firstboot-network.stamp
 else
     echo "info: no files to copy from ${initramfs_firstboot_network_dir}. skipping"
 fi