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

Nokia N900 (RX-51) phone

modulename: board-rx51.ko

and/or

board-rx51-peripherals.ko

and/or

board-rx51-video.ko

configname: CONFIG_MACH_NOKIA_RX51

Linux Kernel Configuration
└─>TI OMAP2/3/4 Specific Features
└─>Nokia N900 (RX-51) phone
In linux kernel since version 2.6.30 (release Date: 2009-06-09)  
source code:
/arch/arm/mach-omap2/


is selected by
CONFIG_SND_OMAP_SOC_RX51