CONFIG_MACH_TX28 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Ka-Ro TX28 module

modulename: mach-tx28.ko

configname: CONFIG_MACH_TX28

Linux Kernel Configuration
└─>Ka-Ro TX28 module
In linux kernel since version 2.6.39 (release Date: 2011-05-18)  
source code:
arch/arm/mach-mxs/mach-tx28.c

depends
CONFIG_ARCH_MXS