This change removes implicit WiFi/BT initialization from startup code.
"main" task is started once essential part of startup code is complete.
This task calls application-provided "int main(void)" function, which can call WiFi/BT init functions if necessary.
Note that with WiFi stack enabled, system_init will reset frequency to 240MHz.
To make this setting useful, esp32-wifi-libs submodule needs to be updated.
