tools: export.sh: better detection and checking of IDF_PATH

1. detect IDF_PATH in zsh, if it is not set
2. if IDF_PATH is set, check that it is valid
3. make sure IDF_PATH is exported, not just set

Related to https://github.com/espressif/esp-idf/issues/3793
Closes https://github.com/espressif/esp-idf/pull/4418
Closes https://github.com/espressif/esp-idf/issues/4770

export.sh

@@ -23,23 +23,54 @@ function idf_export_main() {
 
     if [[ -z "${IDF_PATH}" ]]
     then
-        # If using bash, try to guess IDF_PATH from script location
+        # IDF_PATH not set in the environment.
+        # If using bash or zsh, try to guess IDF_PATH from script location.
+        self_path=""
         if [[ -n "${BASH_SOURCE}" ]]
         then
-            if [[ "$OSTYPE" == "darwin"* ]]; then
-                script_dir="$(realpath_int $BASH_SOURCE)"
-            else
-                script_name="$(readlink -f $BASH_SOURCE)"
-                script_dir="$(dirname $script_name)"
-            fi
-            export IDF_PATH="${script_dir}"
+            self_path="${BASH_SOURCE}"
+        elif [[ -n "${ZSH_VERSION}" ]]
+        then
+            self_path="${(%):-%x}"
         else
-            echo "IDF_PATH must be set before sourcing this script"
+            echo "Could not detect IDF_PATH. Please set it before sourcing this script:"
+            echo "  export IDF_PATH=(add path here)"
             return 1
         fi
+
+        if [[ "$OSTYPE" == "darwin"* ]]; then
+            # convert possibly relative path to absolute
+            script_dir="$(realpath_int ${self_path})"
+            # resolve any ../ references to make the path shorter
+            script_dir="$(cd ${script_dir}; pwd)"
+        else
+            # convert to full path and get the directory name of that
+            script_name="$(readlink -f ${self_path})"
+            script_dir="$(dirname ${script_name})"
+        fi
+        export IDF_PATH="${script_dir}"
+        echo "Setting IDF_PATH to '${IDF_PATH}'"
+    else
+        # IDF_PATH came from the environment, check if the path is valid
+        if [[ ! -d "${IDF_PATH}" ]]
+        then
+            echo "IDF_PATH is set to '${IDF_PATH}', but it is not a valid directory."
+            echo "If you have set IDF_PATH manually, check if the path is correct."
+            return 1
+        fi
+        # Check if this path looks like an IDF directory
+        if [[ ! -f "${IDF_PATH}/tools/idf.py" || ! -f "${IDF_PATH}/tools/idf_tools.py" ]]
+        then
+            echo "IDF_PATH is set to '${IDF_PATH}', but it doesn't look like an ESP-IDF directory."
+            echo "If you have set IDF_PATH manually, check if the path is correct."
+            return 1
+        fi
+
+        # The varible might have been set (rather than exported), re-export it to be sure
+        export IDF_PATH="${IDF_PATH}"
     fi
 
-    old_path=$PATH
+    old_path="$PATH"
 
     echo "Adding ESP-IDF tools to PATH..."
     # Call idf_tools.py to export tool paths