I just uploaded a new raspi3-firmware package which uses ttyS1 on Linux
≥ 4.14, which is the new device name for the UART on the pin header:
https://anonscm.debian.org/cgit/pkg-raspi/raspi3- firmware.git/commit/?id= 4da2e6b1576f0a7513bffe7f958826 93fdadd7ae