Enable local persistent storage

Machine-specific data, such as SSH keys, should not be included in the
immutable root filesystem image, as this would prevent multiple machines
running from the same image.  These few files can be stored locally, on
the SD card on eMMC flash.

The first time a machine boots up using this image, its local storage is
initialized.  This involves creating a new filesystem on the block
device and generating SSH host keys.  Once the storage is initialized,
it is remounted read-only.  All subsequent mounts are read-only as well.

overlay/etc/systemd/system/init-localstorage.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Initialize Local Storage
+DefaultDependencies=no
+After=local-fs-pre.target
+After=run-storage.mount
+Before=etc-ssh.mount
+Before=local-fs.target
+Conflicts=shutdown.target
+ConditionPathExists=!/run/storage/ssh
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/libexec/init-localstorage.sh
+
+[Install]
+WantedBy=run-storage.mount

overlay/etc/systemd/system/run-storage.mount.wants/init-localstorage.service

@@ -0,0 +1 @@
+../init-localstorage.service