Add BSSID filter for testing purposes

Add BSSID filter for testing purposes

wpa_supplicant can now be configured to filter out scan results based
on a BSSID filter. Space-separated set of allowed BSSIDs can be set
with wpa_cli set bssid_filter command. Filtering mechanism can be
disabled by setting this variable to an empty list. When set, only
the BSSes that have a matching entry in this list will be accepted
from scan results.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>