feat(tools): Add backup option to use legacy export script

This feature serves as an immediate fix in case
the new export approach causes issues.
The user can simply set the environment variable `ESP_IDF_LEGACY_EXPORT`,
and the old export script will be used.

export.sh

@@ -1,5 +1,12 @@
 # This script should be sourced, not executed.
 
+# Emergency backup option to use previous export.sh (export_legacy.sh) if the new export approach fails.
+# To use it, set environmental variable like: export ESP_IDF_LEGACY_EXPORT=1
+if [ -n "${ESP_IDF_LEGACY_EXPORT-}" ]; then
+    . ./tools/legacy_exports/export_legacy.sh
+    return $?
+fi
+
 # shellcheck disable=SC2128,SC2169,SC2039,SC3054 # ignore array expansion warning
 if [ -n "${BASH_SOURCE-}" ] && [ "${BASH_SOURCE[0]}" = "${0}" ]
 then