Merge branch 'feature/opt_scan_before_connect' into 'master'

optimize scan before station connecting to AP

1. Store the information of AP(ssid, password, bssid, channel, etc)
    into nvs when station connects to AP successfully. If station
    connects to the same AP next time, it will scan the stored channel of the AP
    first.

2. Add a parameter of channel for scanning before connecting to AP.
    If the channel is set to 0, station will scan full channels. If it
    is set to 1~13, station will only scan the channel.

See merge request !704

components/esp32/include/esp_wifi_types.h

@@ -163,6 +163,7 @@ typedef struct {
     uint8_t password[64];  /**< password of target AP*/
     bool bssid_set;        /**< whether set MAC address of target AP or not. Generally, station_config.bssid_set needs to be 0; and it needs to be 1 only when users need to check the MAC address of the AP.*/
     uint8_t bssid[6];     /**< MAC address of target AP*/
+    uint8_t channel;       /**< channel of target AP. Set to 1~13 to scan starting from the specified channel before connecting to AP. If the channel of AP is unknown, set it to 0.*/
 } wifi_sta_config_t;
 
 typedef union {