As well as adding in the DTLS_PSK wrappers in the appropriate place. add in
coap_dtls_psk_is_supported(), coap_dtls_pki_is_supported(),
coap_dtls_pkcs11_is_supported() and coap_dtls_rpk_is_supported() functions
to support better granularity of what capabilities are available in the
(D)TLS libraries.
Update coap_string_tls_support() to use these new functions.
Uppdated documentation for these new functions.