CONFIG_ECONET is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

EcoNet MIPS family

configname: CONFIG_ECONET

System type
└─>EcoNet MIPS family
In linux kernel since version 2.6.12  
EcoNet EN75xx MIPS devices are big endian MIPS machines used
in XPON (fiber) and DSL applications. They have SPI, PCI, USB,
GPIO, and Ethernet, with optional XPON, DSL, and VoIP DSP cores.
Don't confuse these with the Airoha ARM devices sometimes referred
to as "EcoNet", this family is for MIPS based devices only.