CONFIG_TOSHIBA_RBTX4938 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Toshiba RBTX4938 board

modulename: fixup-rbtx4938.ko

configname: CONFIG_TOSHIBA_RBTX4938

Linux Kernel Configuration
└─>Toshiba RBTX4938 board
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
This Toshiba board is based on the TX4938 processor. Say Y here to
support this machine type

source code:
arch/mips/pci/fixup-rbtx4938.c