Enhance network boot troubleshooting documentation and scripts

Update NETWORK-BOOT-TROUBLESHOOTING.md to clarify the boot process after start4.elf, emphasizing the importance of config.txt settings for kernel and initramfs. Introduce checks for GPU logging and ensure proper configuration for UART. Modify initramfs scripts to improve DHCP lease acquisition and ensure shell output is directed to the serial console. Update ensure-tftpboot-config-kernel-initrd.sh to enforce necessary config settings and link DTB files in serial-prefix directories for better device compatibility.

emmc-provisioning/network-boot-initramfs/init

@@ -15,10 +15,11 @@ mount -t devtmpfs none /dev
 mkdir -p /dev/pts
 mount -t devpts none /dev/pts
 
-# Kernel might have brought up eth0 via ip=dhcp; ensure we have an IP
+# Kernel might have brought up eth0 via ip=dhcp; ensure we have an IP (run in background with timeout so we don't block rescue shell)
 if ! ip addr show | grep -q 'inet .* scope global'; then
   echo "Getting DHCP lease..."
-  udhcpc -f -q -i eth0 -n 2>/dev/null || true
+  ( udhcpc -f -q -i eth0 -n -T 5 2>/dev/null || true ) &
+  sleep 6
 fi
 
 # Allow kernel cmdline to override: provisioning_server=... and rescue mode
@@ -35,7 +36,8 @@ export PROVISIONING_SERVER
 if [ "$RESCUE" -eq 1 ]; then
   echo "=== RESCUE MODE (provisioning_rescue=1) ==="
   echo "Run /rescue-eeprom.sh to mount eMMC and change boot order (rpi-eeprom-config), then reboot."
-  echo "Or run /bin/sh for a shell."
+  # Ensure shell I/O goes to serial console (some setups drop output otherwise)
+  [ -c /dev/console ] && exec </dev/console >/dev/console 2>&1
   exec /bin/sh -i
 fi