CONFIG_DEBUG_NSPIRE_CX_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging via TI-NSPIRE PL011 UART

configname: CONFIG_DEBUG_NSPIRE_CX_UART

Kernel low-level debugging port
└─>Kernel low-level debugging via TI-NSPIRE PL011 UART
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Say Y here if you want kernel low-level debugging support
on TI-NSPIRE CX models.