Add troubleshooting guidance for network boot failures

Introduce a new section in NETWORK-BOOT-TROUBLESHOOTING.md addressing issues where boot stops after start4.elf, detailing necessary config.txt settings for kernel and initramfs. Include instructions for ensuring the presence of required files in the TFTP root and a command to verify configurations on the LXC. Update initrd.img to reflect changes in the network boot process.

emmc-provisioning/scripts/ensure-tftpboot-config-kernel-initrd.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# Ensure TFTP config.txt on the LXC has kernel=kernel8.img and initramfs initrd.img followkernel
+# so the bootloader loads the kernel and initrd (otherwise boot stops after start4.elf).
+# Run on LXC: bash ensure-tftpboot-config-kernel-initrd.sh
+# Or: ssh root@10.20.30.153 'bash -s' < emmc-provisioning/scripts/ensure-tftpboot-config-kernel-initrd.sh
+
+set -e
+TFTP_ROOT="${TFTP_ROOT:-/srv/tftpboot}"
+CONFIG="$TFTP_ROOT/config.txt"
+
+if [[ ! -f "$CONFIG" ]]; then
+  echo "Error: $CONFIG not found."
+  exit 1
+fi
+
+CHANGED=0
+if ! grep -qE '^kernel=kernel8\.img' "$CONFIG" 2>/dev/null; then
+  echo "Adding kernel=kernel8.img to $CONFIG"
+  echo "kernel=kernel8.img" >> "$CONFIG"
+  CHANGED=1
+fi
+if ! grep -qE 'initramfs initrd\.img' "$CONFIG" 2>/dev/null; then
+  echo "Adding initramfs initrd.img followkernel to $CONFIG"
+  echo "" >> "$CONFIG"
+  echo "# Provisioning initramfs (network-boot-initramfs)" >> "$CONFIG"
+  echo "initramfs initrd.img followkernel" >> "$CONFIG"
+  CHANGED=1
+fi
+
+if [[ "$CHANGED" -eq 1 ]]; then
+  echo "Config updated. Ensure $TFTP_ROOT has kernel8.img and initrd.img."
+else
+  echo "Config already has kernel and initramfs lines."
+fi
+grep -E 'kernel|initramfs' "$CONFIG" 2>/dev/null || true
+
+# Ensure serial-prefix dir gets a real copy of config (some TFTP servers don't follow symlinks)
+for serial_dir in "$TFTP_ROOT"/[0-9a-f]*/; do
+  [[ -d "$serial_dir" ]] || continue
+  if [[ -L "$serial_dir/config.txt" ]] || [[ ! -f "$serial_dir/config.txt" ]]; then
+    rm -f "$serial_dir/config.txt"
+    cp "$CONFIG" "$serial_dir/config.txt"
+    echo "Copied config.txt into $(basename "$serial_dir")/ (real file) so device gets full config."
+  fi
+done