Allow different SSL backends to be used (specifically mbedTLS)

[systemcrash]

https://github.com/project-chip/matter-openwrt/blame/71fe2d46f80af07be03eb5382da5e6562d6795bb/README.md#L34-L36

Is it s project requirement to use OpenSSL?

OpenWrt is moving to mbedTLS. Without doing a deeper dive, how flexible is this project SDK on which lib it will use?

[ksperling-apple]

The Matter SDK has a crypto abstraction layer in place and currently supports OpenSSL, BoringSSL, and mbedTLS (or the use of a custom platform implementation). There shouldn't be a problem with supporting mbedTLS when building under OpenWrt.

So there's no requirement to use OpenSSL as such, it's just the default when building on Linux (and I haven't tried to build against the WolfSSL compatibility layer to see if that just works or would require extra effort).

It sounds like medium term we should expose separate package variants with different backends similar to the hosted package.