hostapd: add Multi-AP patches and config options

hostapd: add Multi-AP patches and config options

Cherry-pick Multi-AP commits from uptream:
 9c06f0f6a hostapd: Add Multi-AP protocol support
 5abc7823b wpa_supplicant: Add Multi-AP backhaul STA support
 a1debd338 tests: Refactor test_multi_ap
 bfcdac1c8 Multi-AP: Don't reject backhaul STA on fronthaul BSS
 cb3c156e7 tests: Update multi_ap_fronthaul_on_ap to match implementation
 56a2d788f WPS: Add multi_ap_subelem to wps_build_wfa_ext()
 83ebf5586 wpa_supplicant: Support Multi-AP backhaul STA onboarding with WPS
 66819b07b hostapd: Support Multi-AP backhaul STA onboarding with WPS
 8682f384c hostapd: Add README-MULTI-AP
 b1daf498a tests: Multi-AP WPS provisioning

Add support for Multi-AP to the UCI configuration. Every wifi-iface gets
an option 'multi_ap'. For APs, its value can be 0 (multi-AP support
disabled), 1 (backhaul AP), 2 (fronthaul AP), or 3 (fronthaul + backhaul
AP). For STAs, it can be 0 (not a backhaul STA) or 1 (backhaul STA, can
only associate with backhaul AP).

Also add new optional parameter to wps_start ubus call of
wpa_supplicant to indicate that a Multi-AP backhaul link is required.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>