riotboard: add support for part command

adding support for the part command allows us to find a (possibly) unique
identifier for a partition on a SD card in a specific slot

This is important to allow us a way to determine which root= parameter
to pass to the kernel. Linux unfortunately uses dynamic device numbering
for fixed SD slots, so using root=/dev/mmcblk0p1 only works if you boot
from the lowest numbered physical slot. eMMC is SD4, so trying to boot
from eMMC while there's a card installed in one or more of the other
slots is difficult without using the part command.