[crypto] Fix CASE on some embedded platforms (#7948)
[crypto] Fix CASE on some embedded platforms (#7948)

Establishing a CASE session requires support for
P256Keypair::ECDH_derive_secret() function which was
disabled for platforms which define MBEDTLS_ECP_ALT.
It is quite common on embedded devices equipped with
dedicated crypto chips to provide custom ECP
implementations.
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