7688 用指令切換 sta mode

Ref :  https://medium.com/maker-cup/smart-7688-duo-ap-station-mode-command%E7%9A%84%E5%88%87%E6%8F%9B%E8%88%87%E8%A8%AD%E5%AE%9A-e8da97b7b332#.3hpmjvld7

# uci set wireless.sta.ssid=MyAP       (AP的SSID)
# uci set wireless.sta.encryption=psk2 (加密模式)
# uci set wireless.sta.key=12345678    (AP密碼)
# uci set wireless.sta.disabled=0      (將stationMode打開)
# uci commit
# wifi