## Safety switch

Now that we are fairly confident that (1) the compiler works, (2) it
produces code for our processor, (3) the resulting binary does not
contain any unwanted code; we’d like to build something that will run on
the actual hardware. But first we want to make sure we can go back to a
working firmware, or to flash a different program.

The stock firmware does this by handling a special USB request, upon
receiving which it reboots in ISP DFU mode. But our experimental
programs will not have USB handling until much later.

So we will establish a rule: In any unfamiliar situation, reboot to ISP.

And, in the beginning, every situation is unfamiliar.

So let’s write a function to reboot to ISP, and call it immediately upon
entering `main`.