This introduces KCONFIG_BOARD_CONFIG and KCONFIG_CPU_CONFIG variable for
boards and CPUs (including common directories) to add default
configuration files to be merged. The current approach, as it uses
Makefile.features, would include boards first, not allowing them to
override CPU configurations.
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.
