try root=MMmm hexadecimal notation, I guess there isn't /dev/whatever parser in kernel for all devices. At least N8x0/770 kernel uses root=1f03 for third mtd partition, mmcblk0p3 could be b303 (major 179, minor 3)

also init=/sbin/init (or whatever you use) might be good just in case

and BTW, you can install bootmenu if you don't mind booting initial code from internal flash and pivot_root-ing to new root a bit later, no kernel changes needed then