CONFIG_XMON_DEFAULT is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Enable xmon by default

configname: CONFIG_XMON_DEFAULT

Linux Kernel Configuration
└─>Enable xmon by default
In linux kernel since version 2.6.12  
xmon is normally disabled unless booted with 'xmon=on'.
Use 'xmon=off' to disable xmon init during runtime.

depends
CONFIG_XMON