Add a new method to retrieve active usb keyboard modifiers. Change-Id: Ief6679ce782b58b9ced207f4f27504fb2a517b76 Signed-off-by: Patrick Rudolph <siro@das-labor.org> Reviewed-on: https://review.coreboot.org/18602 Reviewed-by: Martin Roth <martinroth@google.com> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>