CONFIG_ETRAX_SERIAL_PORT0 is not available for thedefaultarchitecture x86.
Result is shown for architecture cris
default or selected kernelversion does not have config value CONFIG_ETRAX_SERIAL_PORT0.
Result is shown for kernelversion 6.7-rc3

Serial port 0 enabled

configname: CONFIG_ETRAX_SERIAL_PORT0

Linux Kernel Configuration
└─>Serial port 0 enabled
In linux kernel since version 2.6.12  
Enables the ETRAX FS serial driver for ser0 (ttyS0)
Normally you want this on. You can control what DMA channels to use
if you do not need DMA to something else.
ser0 can use dma4 or dma6 for output and dma5 or dma7 for input.