armbian: protect armbianEnv.txt against power-loss corruption

- bootscript: use command return values instead of rootdev sentinel
  for corruption detection, add .bak fallback with load/import checks
- common.inc: create armbianEnv.txt.bak at build time (no systemd service)

Author: mingzhangqun

config/bootscripts/boot-seeed-rk35xx.cmd

@@ -21,9 +21,31 @@ test -n "${distro_bootpart}" || distro_bootpart=1
 
 echo "Boot script loaded from ${devtype} ${devnum}:${distro_bootpart}"
 
-if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}armbianEnv.txt; then
-	load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt
-	env import -t ${load_addr} ${filesize}
+# Load armbianEnv.txt with corruption detection and .bak fallback.
+# sed -i + power loss can zero or truncate the file.
+setenv armbian_env_loaded "no"
+if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt; then
+	if env import -t ${load_addr} ${filesize}; then
+		setenv armbian_env_loaded "yes"
+	else
+		echo "WARNING: armbianEnv.txt format error"
+	fi
+fi
+if test "${armbian_env_loaded}" = "no"; then
+	if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt.bak; then
+		echo "Loading armbianEnv.txt.bak as fallback"
+		if env import -t ${load_addr} ${filesize}; then
+			setenv armbian_env_loaded "yes"
+		else
+			echo "ERROR: armbianEnv.txt.bak format error"
+		fi
+	else
+		echo "WARNING: no armbianEnv.txt.bak found"
+	fi
+fi
+# Final safety: restore default rootdev if all sources failed
+if test -z "${rootdev}"; then
+	setenv rootdev "/dev/mmcblk0p1"
 fi
 
 # fdtfile is set by armbianEnv.txt (per-board BOOT_FDT_FILE).

config/sources/vendors/seeed-studio/recomputer-rk35xx-common.inc

@@ -510,3 +510,17 @@ function post_family_tweaks__recomputer_rk35xx_disable_armbian_hardware_optimize
 	chroot_sdcard systemctl --no-reload disable armbian-hardware-optimize.service 2>/dev/null || true
 	rm -f "${SDCARD}"/lib/systemd/system/armbian-hardware-optimize.service
 }
+
+# Create armbianEnv.txt.bak at build time.
+# Protects against sed -i + power loss corruption.
+function pre_umount_final_image__recomputer_armbianenv_backup() {
+	display_alert "$BOARD" "Creating armbianEnv.txt backup" "info"
+
+	local env_file="${MOUNT}/boot/armbianEnv.txt"
+	local bak_file="${MOUNT}/boot/armbianEnv.txt.bak"
+
+	if [[ -f "${env_file}" ]]; then
+		cp -a "${env_file}" "${bak_file}"
+		display_alert "$BOARD" "armbianEnv.txt.bak created"
+	fi
+}