tools: ynl: rename make hardclean -> distclean

The make target to remove all generated files used to be called
"hardclean" because it deleted files which were tracked by git.
We no longer track generated user space files, so use the more
common "distclean" name.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>