Flashing the ubuntu-touch devel channels should print out a warning

This might seem a bit of a hack but we would really need a big disclaimer printed to the users whenever they try to flash one of the ubuntu-touch/devel/* or ubuntu-touch/devel-proposed/* channels. This warning should say something like: "Warning! You are about to flash your phone with a development image - continue at your own risk! There is absolutely no guarantee that most of the basic phone functions will work.". We want to make sure it's perfectly clear for everyone that might have missed our e-mails and docs that these channels are not anything we would recommend using.