usb-dfu: enter program mode in absence of app

When no app has been flashed yet, the frequent resets seem to prevent us
from entering program mode using the reset button.  Enter program mode
if the app stack pointer has not been initialized yet.

Tested-by: smeding
lib/dfu: spell MC HCK MC HCK
lib/dfu: configuration numbers are > 0

Noticed-by: smeding