CONFIG_ETRAXFS_SERIAL is not available for thedefaultarchitecture x86.
Result is shown for architecture cris

Serial-port support

configname: CONFIG_ETRAXFS_SERIAL

Linux Kernel Configuration
└─>Serial-port support
In linux kernel since version 2.6.13 (release Date: 2005-08-28)  
Enables the ETRAX FS serial driver for ser0 (ttyS0)
You probably want this enabled.