[dtls] fix ECDSA configuration for newer mbed TLS versions (#6261)

When using newer versions of mbed TLS, two issues appear when using
`OPENTHREAD_ECDSA`:
- An empty list of configuration hashes is treated as a configuration
error: DTLS fails.
- Identifiers with double underscore are not used anymore: signature
hashes configuration is not included.

This commit addresses both issues.

src/core/meshcop/dtls.cpp

@@ -56,8 +56,8 @@ namespace ot {
 namespace MeshCoP {
 
 const mbedtls_ecp_group_id Dtls::sCurves[] = {MBEDTLS_ECP_DP_SECP256R1, MBEDTLS_ECP_DP_NONE};
-#ifdef MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED
-const int Dtls::sHashes[] = {MBEDTLS_MD_NONE};
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED) || defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
+const int Dtls::sHashes[] = {MBEDTLS_MD_SHA256, MBEDTLS_MD_NONE};
 #endif
 
 Dtls::Dtls(Instance &aInstance, bool aLayerTwoSecurity)
@@ -284,7 +284,7 @@ otError Dtls::Setup(bool aClient)
     if (mCipherSuites[0] == MBEDTLS_TLS_ECJPAKE_WITH_AES_128_CCM_8)
     {
         mbedtls_ssl_conf_curves(&mConf, sCurves);
-#ifdef MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED) || defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
         mbedtls_ssl_conf_sig_hashes(&mConf, sHashes);
 #endif
     }

src/core/meshcop/dtls.hpp

@@ -414,7 +414,7 @@ private:
     uint8_t mPskLength;
 
     static const mbedtls_ecp_group_id sCurves[];
-#ifdef MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED) || defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
     static const int sHashes[];
 #endif