Install mqttdpms

The `mqttdpms` tool will allow the screens to be turned off and on using
Home Assistant.  We'll install it into the rootfs image by copying the
artifact from its Jenkins build into the overlay tree before building
the image.

overlay/etc/systemd/user/mqttdpms.service

@@ -0,0 +1,9 @@
+[Unit]
+Description=MQTT DPMS Control
+
+[Service]
+ExecStart=/usr/local/bin/mqttdpms
+Environment=RUST_LOG=info
+Restart=always
+StandardInput=null
+StandardOutput=journal