Screen displays "DFU mode" when appropriate

From @JamesBaker and @Casey_Chambers in a firmware thread:

Some sort of indication on the display that the machine was in DFU mode would be really useful. As it is, there’s no apparent difference between the Form 1 in its “off” state and the Form 1 in the “DFU” state.

Agreed, and I can totally see why you’d want to confirm this!