I rebuilt socfpga_arria10_socdk_sdmmc.dtb in ./sd_card/sdfs/ using modified files:

   arch/arm/boot/dts/socfpga_arria10_socdk_sdmmc.dts
   arch/arm/boot/dts/socfpga_arria10_socdk.dtsi

When I try booting I now get:

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(179,2)

What is this telling me?

 

I searched online and read something about the boot command in extlinux/extlinux.conf: 'root=/dev/mmcblk0p2'?

The '2' is the problem somehow?