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.
Peripheral configured:
- 5 UARTs for stdio via STLink, Arduino connector, PMOD connector,
STMOD+ connector and ESP-01 connector
- 1 I2C available on Arduino connector and STMOD+ connector
- 2 SPIs available on Arduino connector and PMOD connector