There are two types of commands on Makefile to clean all of made files.
One is make clean, another is make distclean. They are executed on os folder.
This command removes files which are made on build procedure like objects, libraries, .depend, Make.dep and etc.
When configuration is changed, executing this guarantees a re-build from scratch.
This includes make clean inside. Additionally this removes configured files like .config and Make.defs.
When you want to use new or another defconfig, this should be executed before configuring new one.