cmake: Fix passing PYTHON path via CMake variable if using IDF as library

Closes https://github.com/espressif/esp-idf/issues/6285

tools/cmake/build.cmake

@@ -130,9 +130,11 @@ function(__build_init idf_path)
     # Must be global so as to be accessible from any subdirectory in custom projects.
     add_library(__idf_build_target STATIC IMPORTED GLOBAL)
 
-    set_default(python "python")
+    # Set the Python path (which may be passed in via -DPYTHON=) and store in a build property
+    set_default(PYTHON "python")
+    file(TO_CMAKE_PATH ${PYTHON} PYTHON)
+    idf_build_set_property(PYTHON ${PYTHON})
 
-    idf_build_set_property(PYTHON ${python})
     idf_build_set_property(IDF_PATH ${idf_path})
 
     idf_build_set_property(__PREFIX idf)

tools/cmake/project.cmake

@@ -6,12 +6,15 @@ cmake_minimum_required(VERSION 3.5)
 # call.
 include(${CMAKE_CURRENT_LIST_DIR}/idf.cmake)
 
+# legacy variable for compatibility
 set(IDFTOOL ${PYTHON} "${IDF_PATH}/tools/idf.py")
-# Internally, the Python interpreter is already set to 'python'. Re-set here
-# to be absolutely sure.
-set_default(PYTHON "python")
-file(TO_CMAKE_PATH ${PYTHON} PYTHON)
-idf_build_set_property(PYTHON ${PYTHON})
+
+# setting PYTHON variable here for compatibility only, new code should use
+# idf_build_get_property(variable PYTHON)
+idf_build_get_property(PYTHON PYTHON)
+if(NOT PYTHON)
+    message(FATAL_ERROR "Internal error, PYTHON build property not set correctly.")
+endif()
 
 # On processing, checking Python required modules can be turned off if it was
 # already checked externally.