util/futility/Makefile: Update clean target

- Fix clean target to pass if output doesn't exist
- Make sure $(RM) is actually defined

Change-Id: Ibcdb0e329084f58b27c3f53213a237d02c922a51
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/18998
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: build bot (Jenkins)
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
1 file changed