CONFIG_MACH_NOKIA_RX51 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Nokia RX-51 board

modulename: board-rx51.ko

configname: CONFIG_MACH_NOKIA_RX51

Linux Kernel Configuration
└─>Nokia RX-51 board
In linux kernel since version 2.6.30 (release Date: 2009-06-09)  
source code:
/arch/arm/mach-omap2/