Move generic AP functionality implementation into src/ap

Move generic AP functionality implementation into src/ap

This code can be shared by both hostapd and wpa_supplicant and this
is an initial step in getting the generic code moved to be under the
src directories. Couple of generic files still remain under the
hostapd directory due to direct dependencies to files there. Once the
dependencies have been removed, they will also be moved to the src/ap
directory to allow wpa_supplicant to be built without requiring anything
from the hostapd directory.