build system: Probable fix for errors due to bad config bypassing components

build system: Probable fix for errors due to bad config bypassing components

See github #311 https://github.com/espressif/esp-idf/issues/311

Should fix weird compiler/linker bugs where config says something is
enabled, but build system says it is disabled.
Particularly noticeable when WiFi/BT libraries fail to
compile/link despite being enabled.

Underlying cause is configuration file regenerating, but component
Makefiles not reevaluating.

Entirely removes the idea that we don't need to generate config for some
targets (like 'clean'). We need valid config for these targets,
otherwise they don't know which files to clean (etc).