CONFIG_HAVE_AT91_DBGU0 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_HAVE_AT91_DBGU0 (not configurable)

configname: CONFIG_HAVE_AT91_DBGU0

Linux Kernel Configuration
└─>HAVE_AT91_DBGU0
In linux kernel since version 3.1 (release Date: 2011-10-24)