Add network specific BSSID black and white lists

Add network specific BSSID black and white lists

This change adds the configuration options "bssid_whitelist" and
"bssid_blacklist" used to limit the AP selection of a network to a
specified (finite) set or discard certain APs.

This can be useful for environments where multiple networks operate
using the same SSID and roaming between those is not desired. It is also
useful to ignore a faulty or otherwise unwanted AP.

Signed-off-by: Stefan Tomanek <stefan.tomanek@wertarbyte.de>