wpa_supplicant: Add Multi-AP backhaul STA support

wpa_supplicant: Add Multi-AP backhaul STA support

Advertise vendor specific Multi-AP IE in (Re)Association Request frames
and process Multi-AP IE from (Re)Association Response frames if the user
enables Multi-AP fuctionality. If the (Re)Association Response frame
does not contain the Multi-AP IE, disassociate.

This adds a new configuration parameter 'multi_ap_backhaul_sta' to
enable/disable Multi-AP functionality.

Enable 4-address mode after association (if the Association Response
frame contains the Multi-AP IE). Also enable the bridge in that case.
This is necessary because wpa_supplicant only enables the bridge in
wpa_drv_if_add(), which only gets called when an interface is added
through the control interface, not when it is configured from the
command line.

Signed-off-by: Venkateswara Naralasetty <vnaralas@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>