Added usbDelay that polls usb port (for system requests) while delaying.
This is called instead of delay in the button and pin delay functions.
Experimental evaluation gave that the cycle count should be roughly divided
by 28.5.
This diff contains three changes.
1. Make timing isPinCorrect independent of storage.pin, to avoid timing attacks
2. Only update failed PIN counter if the user entered a PIN.
Of course, the fail counter is still incremented, before the PIN is checked.
3. Don't cache the PIN, but just the fact that the PIN was entered. The
cache should be in sync with storage.pin in any case.
