On hardware platforms like ConnectCore 9M2443 the serial driver might create device files like /dev/ttySAC*, for NetSilicon processors they might be created as /dev/ttyNS*.
Some of the startup scripts refer to the standard UART device file names: /dev/ttyS*.
Either select in the serial driver to provide standard names (if driver allows to configure this),
or create the default rootfs which should contain appropriate scripts or symbolic links provided by Digi
or modify your /etc/inittab to match the correct device files names when starting a serial console session on UART.