Makefile.proof: Reorder theorem provers

Prefer Z3 over CVC4, the latter seems to not do anything for the current
code and just wastes CPU cycles.

Change-Id: Ia5e1341e881c5f887452486a318e8e72513f28fb
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/20627
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Adrian-Ken Rueegsegger <ken@codelabs.ch>
diff --git a/Makefile.proof b/Makefile.proof
index 8b06cdc..45523c3 100644
--- a/Makefile.proof
+++ b/Makefile.proof
@@ -11,7 +11,7 @@
 SPARKFLAGS += --mode=$(PROOF_MODE)
 SPARKFLAGS += --report=fail
 SPARKFLAGS += --warnings=error
-SPARKFLAGS += --prover=cvc4,z3 --steps=500 --timeout=1	# FIXME: timeout used because steps seems broken
+SPARKFLAGS += --prover=z3,cvc4 --steps=500 --timeout=1	# FIXME: timeout used because steps seems broken
 
 quote-list = $(subst $(space),$(comma),$(patsubst %,"%",$(1)))