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

Ethernet support

configname: CONFIG_ETRAX_ETHERNET

Linux Kernel Configuration
└─>Ethernet support
In linux kernel since version 2.6.12  
This option enables the ETRAX FS built-in 10/100Mbit Ethernet
controller.

selects
CONFIG_MII