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
