Silence and fail gracefully for clean-up steps

For any clean-up steps, we attempt to clean up but don't fail if
the particular step fails.  This ensures as many clean-up steps get
run as possible even if individually some fail.  E.g.
 $ m clean-linux clobber

The *clobber* still runs even if the *clean-linux* step fails.

Signed-off-by: Ash Charles <ashcharles@gmail.com>