Use /bin/rm to remove /firstboot*. Otherwise rm -i alias is picked
up and can cause issues on boot with the prompts. Fix the read-only
root case with horrible kludge of mounting rw removing the files, then
mounting ro. But since that's no more horrible than the kludge of
using marker files in /. With this change, NanoBSD configs can safely
use /firstboot + growfs to produce minimal images that grow to the
size of the card.