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.
When IDF_PATH is not set by the user, cmake infers the same through
other means and sets it as an environment variable. However, some
sub-invocation of cmake may not see this variable as it is not set
in the parent. This change adds a custom command for exporting IDF_PATH
just before invoking ldgen so that IDF_PATH can be used in Kconfig
without any errors.
