Many simple kconfig configurations are added to boards and brought in with the board name.
This makes a common import of these configs so one only requires adding a BOARD.config without having to alter the makefile.
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.
