hostapd: Add feature to start all interfaces at the same time in sync

hostapd: Add feature to start all interfaces at the same time in sync

When multiple interfaces across mutiple radios are started using a
single instance of hostapd, they all come up at different times
depending upon how long the ACS and HT scan take on each radio. This
will result in stations (that already have the AP profile) associating
with the first interfaces that comes up. For example in a dual band
radio case (2G and 5G) with ACS enabled, 2G always comes up first
because the ACS scan takes less time on 2G and this results in all
stations associating with the 2G interface first.

This feature brings up all the interfaces at the same time. The list of
interfaces specified via hostapd.conf files on the command line are all
marked as sync interfaces. All the interfaces are synchronized in
hostapd_setup_interface_complete().

This feature is turned on with '-S' commmand line option.

Signed-off-by: Srinivasa Duvvuri <sduvvuri@chromium.org>