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>