from the Makefile:
###
# Cleaning is done on three levels.
# make clean Delete most generated files;
# Leave enough to build external modules
# (remove all the object *.o and kernel object files *.ko
# and a few other things)
# make mrproper Delete the current configuration, and all generated files
# make distclean Remove editor backup files, patch leftover files and the like
# (plus remove your config file, the dependency files,
# and everything else that make config creates.)
