make it easier to unbootstrap

[jyio]

We need a method to easily remove BotBrew from the system. For now, the easiest way to completely get rid of BotBrew is

/data/botbrew-basil/init -u
busybox umount -l /data/botbrew-basil
rm -r /data/botbrew-basil

Because busybox may not be available, it might be a good idea to make a native program to do this. Alternatively, just use the busybox that ships with BotBrew.

It might also be useful to spare /home, /root, and/or /etc from deletion. This should be configurable.