hostapd: add UCI option for Multiple BSSID

hostapd: add UCI option for Multiple BSSID

Add an UCI option to enable Multiple BSSID Advertisement. Enabling this
will announce all BSSIDS on a phy in a single beacon frame. The
interface that is brought up first will be the transmitting profile, all
others are non-transmitting profiles and will be advertised in the
Multiple BSSID element in Beacon and Probe Response frames of the first
interface.

This depends on driver and client support. Enabling this will result in
all but the first interface not being visible at all for clients that do
not support it.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>