wpa_supplicant: Enable BGSCAN module

Compile wpa_supplicant with the BGSCAN module enabled. This allows the
user to configure an SSID to use the bgscan module.  This module causes
wpa_supplicant to periodically perform a background scan for additional
access points and switch to the one with the highest signal.  This scan
can be kicked off when the current connection drops below a target
threshold signal strength.