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

Intel HCDDBBVA0 Development Platform (aka Mainstone)

modulename: pxa2xx_mainstone.ko

and/or

mainstone.ko

configname: CONFIG_MACH_MAINSTONE

Linux Kernel Configuration
└─>PXA2xx/PXA3xx-based
└─>Intel PXA2xx/PXA3xx Implementations
└─>Intel HCDDBBVA0 Development Platform (aka Mainstone)
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
source code:
arch/arm/mach-pxa/mainstone.c


is selected by
CONFIG_PCMCIA_PXA2XX