cmake: fix mconf path for existing mconf
cmake: fix mconf path for existing mconf

Fixes https://github.com/espressif/esp-idf/issues/2771
Fixes https://github.com/espressif/esp-idf/issues/2722

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf