Removed `usbDelay(uint32_t cycles)`, added `usbSleep(uint32_t millis)`
The same method signature could cause silent code breakage at runtime,
as opposed to noisy code breakage at compile time which is the better
kind.
Holding confirm button at home screen asks user whether they wish to
lock the TREZOR (clear the cached PIN and passphrase and show the
screensaver). This is identical behaviour to the ClearSession message.
