riscv32-esp-elf toolchain (used for ESP32-C3) can also be used for
ESP32-S2 RISC-V ULP coprocessor.
This removes the riscv-none-embed-gcc toolchain which was originally
used for the ULP, and updates the docs and CMake files to use
riscv32-esp-elf.
Some flags are cleaned up and workarounds removed from CMake toolchain
file.
A problem if the Python interpreter used for idf.py (or set via PYTHON
variable) didn't match
"/usr/bin/env python" (or the associated executable for .py files, on
Windows).
Closes https://github.com/espressif/esp-idf/issues/3160
Possibly also fix for https://github.com/espressif/esp-idf/issues/2936
Adds build system test to catch any future direct execution of Python in
the standard build process.
Ivan Grokhotkov