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/build.sh

@@ -144,13 +144,18 @@ if [ ! -f "$BUILD_DIR/bin/busybox" ] || [ ! -s "$BUILD_DIR/bin/busybox" ]; then
 fi
 chmod +x "$BUILD_DIR/bin/busybox" 2>/dev/null || true
 
-# Busybox applets we need (sh, mount, udhcpc, etc.)
+# Busybox applet symlinks (mount, mkdir, etc.). When building arm64 on x86, busybox cannot be run so --list fails; create symlinks manually.
+APPLETS="sh ash mount umount mkdir cat ip udhcpc sleep echo grep cut awk hostname dd reboot chroot ls rm"
 cd "$BUILD_DIR/bin"
-./busybox --list 2>/dev/null | while read applet; do
-  case "$applet" in
-    sh|ash|mount|umount|mkdir|cat|ip|udhcpc|sleep|echo|grep|cut|awk|hostname|dd|reboot) ln -sf busybox "$applet"; ;;
-  esac
-done
+if ./busybox --list >/dev/null 2>&1; then
+  ./busybox --list | while read applet; do
+    case " $APPLETS " in *" $applet "*) ln -sf busybox "$applet"; ;; esac
+  done
+else
+  for applet in $APPLETS; do
+    [ -e "$applet" ] || ln -sf busybox "$applet"
+  done
+fi
 [ -e sh ] || ln -sf busybox sh
 
 # Build cpio (gzip)