overlay: 15coreos-copy-firstboot-network: handle race condition

Add short while loop to handle a race condition where
After=dev-disk-by\x2dlabel-boot.device doesn't seem to be sufficient
always.

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

@@ -4,17 +4,39 @@ set -euo pipefail
 # For a description of how this is used see coreos-copy-firstboot-network.service
 
 bootmnt=/mnt/boot_partition
+bootdev=/dev/disk/by-label/boot
 firstboot_network_dir_basename="coreos-firstboot-network"
 initramfs_firstboot_network_dir="${bootmnt}/${firstboot_network_dir_basename}"
 initramfs_network_dir="/run/NetworkManager/system-connections/"
 realroot_firstboot_network_dir="/boot/${firstboot_network_dir_basename}"
 
+# Wait for up to 5 seconds for the boot device to be available
+# The After=...*boot.device in the systemd unit should be enough
+# but we saw races in CI where this didn't seem to always be true:
+#
+#   [    4.045181] systemd[1]: Found device /dev/disk/by-label/boot.
+#   [  OK  ] Found device /dev/disk/by-label/boot
+#   [    4.051500] systemd[1]: Starting Copy CoreOS Firstboot Networking Config...
+#         Starting  Copy CoreOS Firstboot Networking Config
+#   [    4.060573]  vda: vda1 vda2 vda3 vda4
+#   [    4.063296] coreos-copy-firstboot-network[479]: mount: /mnt/boot_partition: special device /dev/disk/by-label/boot does not exist.
+#
+for x in {1..5}; do
+    if [ -e ${bootdev} ]; then
+        echo "info: found ${bootdev}" 
+        break
+    else
+        echo "info: waiting for ${bootdev} to show up..."
+        sleep 1
+    fi
+done
+
 # Mount /boot. Note that we mount /boot but we don't unmount boot because we
 # are run in a systemd unit with MountFlags=slave so it is unmounted for us.
 mkdir -p ${bootmnt}
 # mount as read-only since we don't strictly need write access and we may be
 # running alongside other code that also has it mounted ro
-mount -o ro /dev/disk/by-label/boot ${bootmnt}
+mount -o ro ${bootdev} ${bootmnt}
 
 if [ -n "$(ls -A ${initramfs_firstboot_network_dir} 2>/dev/null)" ]; then
     # Clear out any files that may have already been generated from