CONFIG_AT91_DEBUG_LL_DBGU1 is not available for the selected architecture x86.
Result is shown for architecture arm

Kernel low-level debugging on 9263 and 9g45

configname: CONFIG_AT91_DEBUG_LL_DBGU1

Kernel low-level debugging port
└─>Kernel low-level debugging on 9263 and 9g45
In linux kernel since version 3.1 (release Date: 2011-10-24)
selects
CONFIG_DEBUG_AT91_UART

depends
CONFIG_HAVE_AT91_DBGU1 or CONFIG_ARCH_AT91 |
| CONFIG_SOC_AT91SAM9 or CONFIG_SOC_SAMA5