Extract the config files related stuff from the kconfig stuff
- create config/config.mk, with all the config.gen/ rules
- the only common points between config files and configurators:
- the top-level config file to include, stored in KCONFIG_TOP
- the config_file rules, which makes config files available to configurators
- dependency-files are renamed from %.d to %.dep (.d is reserved for directories)
- a few eye-candy here and there