Makefile: cleanup *clean targets

As discussed on the list, fixup the *clean targets, so we now have:

clean: remove all generated files (E.G. everything in output/)
distclean: same as clean, but also remove the kconfig build and .config

Also tweak 'make help' output to match. This also merges the two
different clean / distclean targets we had depending on if there was
a .config present or not (most variables are not defined for !.config
case, so E.G. the clean target ends up being a noop).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>