CONFIG_MACH_ASPEED_G5 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Aspeed SoC 5th Generation

configname: CONFIG_MACH_ASPEED_G5

Linux Kernel Configuration
└─>Aspeed BMC architectures
└─>Aspeed SoC 5th Generation
In linux kernel since version 4.1 (release Date: 2015-06-21)  
Say yes if you intend to run on an Aspeed ast2500 or similar
fifth generation Aspeed BMCs.