hostapd: review request for mbedtls EAP-TLS 1.3 support

[Karsten Sperling]

Hi Felix, (also CC Hauke since he looked at #25825 a few weeks ago)

Could I please ask for a review of https://github.com/openwrt/openwrt/pull/26069 ?

In the current mbedtls backend implementation, TLS 1.3 is broken both on the AP and STA side. The PR fixes this by implementing the TLS 1.3 keying material export, and also addresses some structural bugs in how the backend manages TLS contexts that manifest when EAP-TLS is configured with different settings on multiple BSSes.

With these changes a number of hwsim tests that previously failed or were skipped now pass, including in mixed configurations (mbedtls AP / openssl STA and vice versa). All TLS 1.3 tests pass (excluding OCSP and session resumption tests, since those features are unsupported). I have also been testing these patches on device in the context of https://github.com/project-chip/matter-openwrt.

It would be great to see EAP-TLS 1.3 support for the default TLS backend land in time for the next major release :-)

Also open, if anyone has time to look, some small independent fixes that fell out of this work:

https://github.com/openwrt/openwrt/pull/25825 standalone RADIUS server fixes 
https://github.com/openwrt/openwrt/pull/25827 rfkill in non-multicall hostapd
https://github.com/openwrt/openwrt/pull/26435 restore original scope of CONFIG_CTRL_IFACE_MIB
https://github.com/openwrt/openwrt/pull/26434 missing UCODE_SUPPORT / CONFIG_DPP ifdefs

Thanks, Karsten