CONFIG_AT91_DEBUG_LL_DBGU0 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging on rm9200, 9260/9g20, 9261/9g10 and 9rl

configname: CONFIG_AT91_DEBUG_LL_DBGU0

Kernel low-level debugging port
└─>Kernel low-level debugging on rm9200, 9260/9g20, 9261/9g10 and 9rl
In linux kernel since version 3.1 (release Date: 2011-10-24)  

depends
CONFIG_HAVE_AT91_DBGU0 or CONFIG_ARCH_AT91 |
| CONFIG_SOC_AT91RM9200 or CONFIG_SOC_AT91SAM9