CONFIG_MMAPPER is not available for thedefaultarchitecture x86.
Result is shown for architecture um

iomem emulation driver

modulename: mmapper_kern.ko

configname: CONFIG_MMAPPER

Linux Kernel Configuration
└─>UML-specific options
└─>iomem emulation driver
In linux kernel since version 2.6.12  
This driver allows a host file to be used as emulated IO memory inside
UML.

source code:
arch/um/drivers/mmapper_kern.c