hostapd: add support for WPS pushbutton station

hostapd: add support for WPS pushbutton station

similar to hostapd, also add a ubus interface for wpa_supplicant
which will allow handling WPS push-button just as it works for hostapd.
In order to have wpa_supplicant running without any network
configuration (so you can use it to retrieve credentials via WPS),
configure wifi-iface in /etc/config/wireless:

  config wifi-iface 'default_radio0'
      option device 'radio0'
      option network 'wwan'
      option mode 'sta'
      option encryption 'wps'

This section will automatically be edited if credentials have
successfully been acquired via WPS.

Size difference (mips_24kc): roughly +4kb for the 'full' variants of
wpa_supplicant and wpad which do support WPS.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>