cmake: Add ESP_PLATFORM in idf_set_variables() as well

(Handles case where idf.py is not being used.)

Ref https://github.com/espressif/esp-idf/pull/2601

tools/cmake/idf_functions.cmake

@@ -62,6 +62,8 @@ macro(idf_set_variables)
 
     set(IDF_PROJECT_PATH "${CMAKE_SOURCE_DIR}")
 
+    set(ESP_PLATFORM 1 CACHE BOOL INTERNAL)
+
     spaces2list(IDF_COMPONENT_DIRS)
     spaces2list(IDF_COMPONENTS)
     spaces2list(IDF_COMPONENT_REQUIRES_COMMON)