curl: fix interface with mbedtls_sha256

Between mbedtls 2.6.0 and 2.7.0, the void returning mbedtls_MODULE* functions
were deprecated in favor of functions returning an int error code.  Use
the new function mbedtls_sha256_ret().

Signed-off-by: Russell Senior <russell@personaltelco.net>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

package/network/utils/curl/patches/320-mbedtls_dont_use_deprecated_sha256_function.patch

@@ -0,0 +1,11 @@
+--- a/lib/vtls/mbedtls.c
++++ b/lib/vtls/mbedtls.c
+@@ -1029,7 +1029,7 @@ static void Curl_mbedtls_sha256sum(const
+                                     size_t sha256len UNUSED_PARAM)
+ {
+   (void)sha256len;
+-  mbedtls_sha256(input, inputlen, sha256sum, 0);
++  mbedtls_sha256_ret(input, inputlen, sha256sum, 0);
+ }
+ 
+ static void *Curl_mbedtls_get_internals(struct ssl_connect_data *connssl,