This feature is only used to blacklist stdio via CDC ACM.
Introduce the `highlevel_stdio` feature instead to indicate
that stdio functionality requires a running thread to print
anything
The private parts need USB definitions (and are thus preferably used
from USB_H_USER_IS_RIOT_INTERNAL compilation units). Functions like
usb_board_reset_in_bootloader do not depend on USB headers for their
definitions and are fair game throughout the application even for
generic RIOT USB devices.
