Add helper function to check whether a CRT msg is expected
Add helper function to check whether a CRT msg is expected

This commit adds a helper function `ssl_parse_certificate_coordinate()`
which checks whether a `Certificate` message is expected from the peer.

The logic is the following:
- For ciphersuites which don't use server-side CRTs, no Certificate
  message is expected (neither for the server, nor the client).
- On the server, no client certificate is expected in the following cases:
  * The server server didn't request a Certificate, which is controlled
    by the `authmode` setting.
  * A RSA-PSK suite is used; this is the only suite using server CRTs
    but not allowing client-side authentication.

ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls