Missing boot2 should trigger error message

[jannic]

While doing some experiments, I accidentally removed the code which includes boot2 into the firmware (pub static BOOT2_FIRMWARE: [u8; 256] = ...)

This didn't cause any warning. The resulting firmware just contained zeros where boot2 should have been. Needless to say that the firmware didn't boot.

In hindsight this is obvious, but at first I had no idea what went wrong. It would be nice if the build could fail in this case, or at least print a warning.