CONFIG_ETRAX_SERIAL is not available for the selected architecture x86.
Result is shown for architecture cris

Serial-port support

modulename: crisv10.ko

configname: CONFIG_ETRAX_SERIAL

Linux Kernel Configuration
└─>Serial-port support
In linux kernel since version 2.6.12 )
Enables the ETRAX 100 serial driver for ser0 (ttyS0)
You probably want this enabled.
source code:
drivers/tty/serial/crisv10.c