From: Sam Ravnborg <sam@ravnborg.org>

Previously 'make clean' deleted all automatically generated files.  The
following patch revert this behaviour, and now 'make clean' leaves enough
behind to allow external modules to be built.

The cleaning is now done in three steps:

make clean     - delete everything not needed for building external modules
make mrproper  - delete all generated files, including .config
make distclean - delete all temporary files such as *.orig, *~, *.rej etc.

This fixes reports about nvidia and vmware build issues.