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>
diff --git a/util/futility/Makefile b/util/futility/Makefile
index 36f2b31..cce5da6 100644
--- a/util/futility/Makefile
+++ b/util/futility/Makefile
@@ -1,5 +1,6 @@
 top ?= $(abspath ../..)
 objutil ?= $(top)/util
+RM ?= rm
 
 HOSTCC ?= $(CC)
 VBOOT_SOURCE ?= $(top)/3rdparty/vboot
@@ -9,8 +10,8 @@
 
 .PHONY: clean
 clean:
-	$(RM) $(objutil)/futility/futility
-	$(RM) -r $(objutil)/futility/build
+	$(RM) -f $(objutil)/futility/futility
+	$(RM) -rf $(objutil)/futility/build
 
 distclean: clean