hostapd: add ubus reload

Add ubus interface to hostapd and wpa_supplicant to allow dynamically
reloading wiface configuration without having to restart the hostapd
process.
As a consequence, both hostapd and wpa_supplicant are now started
persistently on boot for each wifi device in the system and then
receive ubus calls adding, modifying or removing interface
configuration.
At a later stage it would be desirable to reduce the services to one
single instance managing all radios.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>