Turn MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE configuration option into a runtime option

[ronald-cron-arm]

Context

The configuration option MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE allows during TLS handshake to select the ciphersuite according to the client's preferences rather than the server's preferences.

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Rationale

The capability provided by the option seems to be useful and used (see #4244 for example) but the code size reduction is marginal thus the decision to turn it to a runtime option.

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Work items for 3.0

• Remove MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE from config.h and all references to it in the code base.
• Add a "respect client preference" SSL runtime option in mbedtls_ssl_config and the API to enable/disable it (something like void mbedtls_ssl_conf_respect_client_preference( mbedtls_ssl_config *conf, int enable ) it seems.
• Adapt the code that is guarded by MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE.