Startup flow refactoring

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.

components/esp32/include/esp_wifi.h

@@ -136,27 +136,6 @@ typedef enum {
     WIFI_SECOND_CHAN_BELOW,     /**< the channel width is HT40 and the second channel is below the primary channel */
 } wifi_second_chan_t;
 
-/**
-  * @brief     startup WiFi driver and register application specific callback function
-  *
-  * @attention 1. This API should be called in application startup code to init WiFi driver
-  * @attention 2. The callback function is used to provide application specific WiFi configuration,
-  *               such as, set the WiFi mode, register the event callback, set AP SSID etc before
-  *               WiFi is startup
-  * @attention 3. Avoid to create application task in the callback, otherwise you may get wrong behavior
-  * @attention 4. If the callback return is not ESP_OK, the startup will fail!
-  * @attention 5. Before this API can be called, system_init()/esp_event_init()/tcpip_adapter_init() should
-  *               be called firstly
-  *
-  * @param  wifi_startup_cb_t cb : application specific callback function
-  * @param  void *ctx : reserved for user
-  *
-  * @return ESP_OK : succeed
-  * @return others : fail
-  */
-typedef esp_err_t (* wifi_startup_cb_t)(void *ctx);
-
-esp_err_t esp_wifi_startup(wifi_startup_cb_t cb, void *ctx);
 
 typedef struct {
     void    *event_q;                 /**< WiFi event q handler, it's a freeRTOS queue */