CONFIG_MIPS_COBALT is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Support for Cobalt Server (EXPERIMENTAL)

modulename: fixup-cobalt.ko

configname: CONFIG_MIPS_COBALT

Linux/MIPS Kernel Configuration
└─>Machine selection
└─>Support for Cobalt Server (EXPERIMENTAL)
In linux kernel since version 2.6.12  
source code:
arch/mips/pci/fixup-cobalt.c

depends
CONFIG_EXPERIMENTAL