CONFIG_MBX is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc
default or selected kernelversion does not have config value CONFIG_MBX.
Result is shown for kernelversion 5.4.123

MBX

configname: CONFIG_MBX

8xx Machine Type
└─>MBX
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
MBX is a line of Motorola single-board computer based around the
MPC821 and MPC860 processors, and intended for embedded-controller
applications. Say Y here to support these boards directly.