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
899a5d223f