TinyCrypt SSL: Adapt calculation of maximum PMS size in ssl.h

ssl.h contains a dummy union of fields each large enough to
hold the PMS for a particular ciphersuite. In particular, for
pure-ECDH ciphersuites, it contains a field large enough to
hold the ECDH shared secret in any of the enabled curves.

So far, this upper bound was unconditionally chosen to be
MBEDTLS_ECP_MAX_BYTES from the ECP module.

With the introduction of TinyCrypt as an alternative implementation
for ECDH, we need to
- guard the use of MBEDTLS_ECP_MAX_BYTES because MBEDTLS_ECP_C
  is no longer implied by the surrounding MBEDTLS_KEY_EXCHANGE_XXX
  guards
- add another field which contains the maximum length of shared
  ECDH secrets for curves supported by TinyCrypt.

include/mbedtls/ssl.h

@@ -46,6 +46,10 @@
 #include "ecdh.h"
 #endif
 
+#if defined(MBEDTLS_USE_TINYCRYPT)
+#include "tinycrypt/ecc.h"
+#endif
+
 #if defined(MBEDTLS_ZLIB_SUPPORT)
 
 #if defined(MBEDTLS_DEPRECATED_WARNING)
@@ -510,8 +514,13 @@ union mbedtls_ssl_premaster_secret
     defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)  || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED)     || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED)
+#if defined(MBEDTLS_ECDH_C)
     unsigned char _pms_ecdh[MBEDTLS_ECP_MAX_BYTES];    /* RFC 4492 5.10 */
 #endif
+#if defined(MBEDTLS_USE_TINYCRYPT)
+    unsigned char _pms_ecdh_uecc[ NUM_ECC_BYTES ];
+#endif
+#endif
 #if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)
     unsigned char _pms_psk[4 + 2 * MBEDTLS_PSK_MAX_LEN];       /* RFC 4279 2 */
 #endif