Fix usb debug dongle support

Fix usb debug dongle support

- move enable_usbdebug() declaration to usbdebug.h
- reinitialize debug driver in ramstage, as copying the data
  structure from romstage doesn't work right now. This way of copying
  data from romstage to ramstage is really board/cpu specific, and is
  likely to break often. So don't do it.

Change-Id: I394678ded6679c1803e29eb691b926182bdcab68
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/355
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>