Enhance eMMC provisioning scripts and documentation: add troubleshooting section for device connection issues, update flash script to prevent concurrent runs, and improve logging. Adjust deployment scripts to verify presence of boot files in mass-storage-gadget.

chromium-setup/emmc-provisioning/host/cm4-flash.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=CM4 eMMC provisioning (rpiboot + backup/deploy)
+# Run after udev has settled; do not block boot
+After=systemd-udevd.service
+DefaultDependencies=yes
+
+[Service]
+Type=oneshot
+# Delay so USB device is enumerated and udev permissions applied before we run
+ExecStartPre=/bin/sleep 5
+ExecStart=/opt/cm4-provisioning/flash-emmc-on-connect.sh
+# Run as root; flash script logs to /var/lib/cm4-provisioning/flash.log
+User=root
+StandardOutput=journal
+StandardError=journal
+# Allow long run (rpiboot + device scan + wait for user choice)
+TimeoutStartSec=900
+
+[Install]
+# Only started by udev trigger, not at boot
+WantedBy=multi-user.target