CONFIG_E300C3_CPU is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

e300c3 (831x)

configname: CONFIG_E300C3_CPU

CPU selection
└─>e300c3 (831x)
In linux kernel since version 3.10 (release Date: 2013-06-30)  
depends
CONFIG_PPC_BOOK3S_32
NOT CONFIG_CC_IS_CLANG