CONFIG_TOSHIBA_RBTX4939 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Toshiba RBTX4939 board

configname: CONFIG_TOSHIBA_RBTX4939

Linux Kernel Configuration
└─>Toshiba RBTX4939 board
In linux kernel since version 2.6.28 (release Date: 2008-12-24)  
This Toshiba board is based on the TX4939 processor. Say Y here to
support this machine type