Not having recent Python versions installed leads to unclear error in detect_python.sh (IDFGH-6929)

[fpedd]

Environment

• Development Kit: n.a.
• IDF version: v5.0-dev-1959-g4cdf2459f0
• Build System: idf.py
• Operating System: Linux, Ubuntu 18.04
• Python 3.6

Problem Description

When only having Python versions "older" than 3.7 installed the tools/detect_python.sh script throws some very cryptic error message.

Expected Behavior

Some meaningful error message about outdated Python version.

Actual Behavior

esp-idf/tools/detect_python.sh: line 24: ESP_PYTHON: unbound variable

Steps to reproduce

1. Clone the latest master
2. Only have Python 3.6 installed
3. Try to install IDF with install.sh

Cause and suggested fix

I believe this is caused by the recent changes in addd647 to the file. Reintroducing ESP_PYTHON=python at the top of the file should fix the issue. It will print the expected error message RuntimeError: ESP-IDF supports Python 3.7 or newer but you are using Python 2.7.17. Please upgrade your installation as described in the documentation.

[dobairoland]

@fabianpedd Thank you for letting us know about this. We will fix it soon.