core: mbedtls: enable MBEDTLS_ECDH_LEGACY_CONTEXT

Compared to Mbed TLS 2.16.5, version 2.22.0 has a different ECDH
context structure (struct mbedtls_ecdh_context). A macro is provided to
support legacy code; let's use it.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/include/mbedtls_config_kernel.h b/lib/libmbedtls/include/mbedtls_config_kernel.h
index 826850c..51a78f3 100644
--- a/lib/libmbedtls/include/mbedtls_config_kernel.h
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -110,6 +110,7 @@
 #define MBEDTLS_ECP_C
 #define MBEDTLS_ECDSA_C
 #define MBEDTLS_ECDH_C
+#define MBEDTLS_ECDH_LEGACY_CONTEXT
 #endif
 
 #endif /*CFG_CRYPTOLIB_NAME_mbedtls*/