You should try to use /dev/ttyS1, and you will need to match the baudrate with the baudrate used by the firmware in question. If you're using the firmware that was shipped on the drone it should be 1500000, otherwise it will depend on how it was compiled.
/dev/ttyS1 is what I tried to do at first, but when I run roslaunch mavros px4.launch fcu_url:=/dev/ttyS0 it returns no errors, even though it ran and all the topics were listed, nothing is actually published because the FCU is not connected based on the diagnostic. When I run roslaunch mavros px4.launch fcu_url:=/dev/ttyS1 ROS simply tells me there is a I/O error and the node has to shutdown.
Moreover, following a link on the Intel Aero Github Wiki, it pointed me here: 04 Autonomous drone programming in Python · guermonprez/IntelAero1 Wiki · GitHub where it suggested to use /dev/ttyS0 on Ubuntu 16.