Fixed an issue when configuration file options partly ignored when `--project-conf` // Resolve #3034 (#3055)

* Fixed an issue when configuration file options partly ignored when using custom ``--project-conf`` // Resolve #3034

* Py2 compatible makedirs

* Fix circle dependency

* Fix broken import in test examples

* Fix history

* Remove YAPF markers

* PyLint fix

* Fix invalid project conf path

* Move PIO Core to the root on Windows, issue with long CPPPATHs

* Respect global PLATFORMIO_BUILD_CACHE_DIR env var

* Fix Appveyor paths

* Minor changes

tests/test_projectconf.py

@@ -112,6 +112,15 @@ def test_warnings(config):
         config.validate(["non-existing-env"])
 
 
+def test_defaults(config):
+    assert config.get_optional_dir("core") == os.path.join(
+        os.path.expanduser("~"), ".platformio"
+    )
+    assert config.get_optional_dir("build_cache") == os.environ.get(
+        "PLATFORMIO_BUILD_CACHE_DIR"
+    )
+
+
 def test_sections(config):
     with pytest.raises(ConfigParser.NoSectionError):
         config.getraw("unknown_section", "unknown_option")