CONFIG_MACH_STMP378X_DEVB is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support STMP378x_devb Platform

modulename: mach-stmp378x_devb.ko

configname: CONFIG_MACH_STMP378X_DEVB

Linux Kernel Configuration
└─>Support STMP378x_devb Platform
In linux kernel since version 3 (release Date: 2011-07-21)  
Include support for STMP378x-devb platform. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-mxs/mach-stmp378x_devb.c

depends
CONFIG_ARCH_MXS