CONFIG_M68000 is not available for thedefaultarchitecture x86.
Result is shown for architecture m68k

MC68000

configname: CONFIG_M68000

Linux Kernel Configuration
└─>MC68000
In linux kernel since version 3 (release Date: 2011-07-21)  
The Freescale (was Motorola) 68000 CPU is the first generation of
the well known M68K family of processors. The CPU core as well as
being available as a stand alone CPU was also used in many
System-On-Chip devices (eg 68328, 68302, etc). It does not contain
a paging MMU.