CONFIG_MACH_STM32MP157 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

STMicroelectronics STM32MP157

configname: CONFIG_MACH_STM32MP157

Linux Kernel Configuration
└─>STMicroelectronics STM32 family
└─>STMicroelectronics STM32MP157
In linux kernel since version 3.10 (release Date: 2013-06-30)