Update kconfiglib to upstream version and replace mconf-idf

Special thanks to @ulfalizer for the helpful suggestions regarding
kconfiglib.

"rsource" option is available for relative path includes
Closes https://github.com/espressif/esp-idf/issues/4064

tools/ldgen/sdkconfig.py

@@ -26,6 +26,8 @@ except Exception:
     sys.path.insert(0, kconfig_new_dir)
     import kconfiglib
 
+import confgen
+
 
 class SDKConfig:
     """
@@ -47,6 +49,7 @@ class SDKConfig:
     OPERATOR = oneOf(["=", "!=", ">", "<", "<=", ">="])
 
     def __init__(self, kconfig_file, sdkconfig_file):
+        confgen.prepare_source_files()
         self.config = kconfiglib.Kconfig(kconfig_file)
         self.config.load_config(sdkconfig_file)