FST L2DA RX forwarding: AP default interface supported

Introduced new INI group-level parameter: l2da_ap_default_ifname. This
parameter is only valid for AP side of L2DA based MUX and it instructs
fstman to set L2DA default slave to l2da_ap_default_ifname in order to
support 802.11ac WDS.

Change-Id: I5fbc0a7c5219f3a790cc33a61fcfab86e45db80a