Hi Niklas,
The default kernel configuration has support for SPI. You can validate this by looking in /proc/mtd to see the flash partitions and you can run "hexdump -c /dev/mtd0r0" to see the content of SPI-NOR.
It seems that you're asking about userspace SPI access though (/dev/spi...). This isn't included in the standard kernel configuration, and you'd need to enable it by running "bitbake -c menuconfig linux-boundary".
I wonder why you're doing this though. Are you hooking up a different SPI device through other pins? If so, you'll need to change the pin-muxing for the SPI and chip-select pins and add another SPI bus to the system as shown here:
linux-imx6/arch/arm/mach-mx6/board-mx6_nitrogen6x.c at boundary-imx_3.0.35_4.1.0 · boundarydevices/l...
If you're doing this level of kernel hacking, I'd strongly recommend building the kernel outside of Yocto, so that you have a proper git repository backing your efforts.