hostapd: add external registrar support

Setting wireless.@wifi-iface[N].ext_registrar=1 will enable UPNP
advertising and add an external registrar to the interface this vif
belongs to (br-lan if the vif is included in the LAN bridge). By
enabling this we append upnp_iface=xxx to the hostapd config file.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>

SVN-Revision: 38338