Hi,
some thoughts: Is the postfix user allowed to read /etc/sasl/smtpd.conf? You could set cyrus_sasl_config_path to /etc/sasl/. How are the access rights of the postfix user to your sasldb-file (/etc/sasldb2?)? In my machines (mostly debian 8) i have the postfix user in sasl group. And, as Viktor mentioned, look if your postfix is build with sasl support. Willi