u-boot DFU needs to block console access while in DFU mode

[laforge@…]

sine DFU does all kinds of strange things (such as doing device i/o) from USB
interrupt context, we should restrict interactive access to the bootloader while
dfu_state != DFU_STATE_appIDLE.

usbtty access is disabled anyway, since the interface and endpoint descriptors
for CDC ACM do not exist in DFU mode. After a successful DOWNLOAD, or to be
more precise: As soon as the USB host issues a USB reset, we switch back to
runtime mode (with usbtty) support anyway.

So this is really mainly about the serial port, which the standard user hasn't
access to anyway.