bootscript: rename armbianEnv.txt.bak fallback to .dist

Per review feedback (#9953), the build-time fallback file
must not collide with user backups. ".bak" is a common name users pick
when editing armbianEnv.txt themselves, and it is easily removed during
routine cleanup of "obsolete" backup files.

Rename the fallback to ".dist" (distribution default), which signals a
build-shipped asset rather than a user backup, and matches the upstream
convention of shipping a default armbianEnv.txt under a distinguished
name.

Updates both the bootscript consumer and the build hook that creates
the file.

Author: mingzhangqun

config/bootscripts/boot-seeed-rk35xx.cmd

@@ -21,7 +21,7 @@ test -n "${distro_bootpart}" || distro_bootpart=1
 
 echo "Boot script loaded from ${devtype} ${devnum}:${distro_bootpart}"
 
-# Load armbianEnv.txt with corruption detection and .bak fallback.
+# Load armbianEnv.txt with corruption detection and .dist fallback.
 # Power loss can fill the file with 0xFF (eMMC erased block) or ^@ (NUL,
 # on other storage media) which passes "env import -t" without error but
 # imports zero variables. Clear rootdev before import; if it remains
@@ -31,8 +31,8 @@ if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv
 	env import -t ${load_addr} ${filesize}
 fi
 if test -z "${rootdev}"; then
-	if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt.bak; then
-		echo "WARNING: armbianEnv.txt corrupt, loading .bak fallback"
+	if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt.dist; then
+		echo "WARNING: armbianEnv.txt corrupt, loading .dist fallback"
 		setenv rootdev
 		env import -t ${load_addr} ${filesize}
 	fi

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

@@ -511,16 +511,16 @@ function post_family_tweaks__recomputer_rk35xx_disable_armbian_hardware_optimize
 	rm -f "${SDCARD}"/lib/systemd/system/armbian-hardware-optimize.service
 }
 
-# Create armbianEnv.txt.bak at build time.
+# Create armbianEnv.txt.dist at build time.
 # Protects against sed -i + power loss corruption.
 function pre_umount_final_image__recomputer_armbianenv_backup() {
-	display_alert "$BOARD" "Creating armbianEnv.txt backup" "info"
+	display_alert "$BOARD" "Creating armbianEnv.txt.dist fallback" "info"
 
 	local env_file="${MOUNT}/boot/armbianEnv.txt"
-	local bak_file="${MOUNT}/boot/armbianEnv.txt.bak"
+	local dist_file="${MOUNT}/boot/armbianEnv.txt.dist"
 
 	if [[ -f "${env_file}" ]]; then
-		cp -a "${env_file}" "${bak_file}"
-		display_alert "$BOARD" "armbianEnv.txt.bak created"
+		cp -a "${env_file}" "${dist_file}"
+		display_alert "$BOARD" "armbianEnv.txt.dist created"
 	fi
 }