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

Intel HCDDBBVA0 Development Platform

modulename: mainstone.ko

configname: CONFIG_MACH_MAINSTONE

Select target board
└─>Intel HCDDBBVA0 Development Platform
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
source code:
arch/arm/mach-pxa/mainstone.c
selects
CONFIG_PXA27x

is selected by
CONFIG_MTD_MAINSTONE