hostapd: Support Multi-AP backhaul STA onboarding with WPS

hostapd: Support Multi-AP backhaul STA onboarding with WPS

The Wi-Fi Alliance Multi-AP Specification v1.0 allows onboarding of a
backhaul STA through WPS. To enable this, the WPS Registrar offers a
different set of credentials (backhaul credentials instead of fronthaul
credentials) when the Multi-AP subelement is present in the WFA vendor
extension element of the WSC M1 message.

Add new configuration options to specify the backhaul credentials for
the hostapd internal registrar: multi_ap_backhaul_ssid,
multi_ap_backhaul_wpa_psk, multi_ap_backhaul_wpa_passphrase. These are
only relevant for a fronthaul SSID, i.e., where multi_ap is set to 2 or
3. When these options are set, pass the backhaul credentials instead of
the normal credentials when the Multi-AP subelement is present.

Ignore the Multi-AP subelement if the backhaul config options are not
set. Note that for an SSID which is fronthaul and backhaul at the same
time (i.e., multi_ap == 3), this results in the correct credentials
being sent anyway.

The security to be used for the backaul BSS is fixed to WPA2PSK. The
Multi-AP Specification only allows Open and WPA2PSK networks to be
configured. Although not stated explicitly, the backhaul link is
intended to be always encrypted, hence WPA2PSK.

To build the credentials, the credential-building code is essentially
copied and simplified. Indeed, the backhaul credentials are always
WPA2PSK and never use per-device PSK. All the options set for the
fronthaul BSS WPS are simply ignored.

Signed-off-by: Davina Lu <ylu@quantenna.com>
Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Marianna Carrera <marianna.carrera.so@quantenna.com>