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 it is set to 1~13, station will scan starting from the channel.
If the channel of AP is unknown, set it to 0.
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