Provide a way to redirect output of fabric-tna build to dev null

Change-Id: Ib4b8787daef193e6c4af09fb0fa00338a0970438
diff --git a/Makefile b/Makefile
index 305bc77..76f66bf 100644
--- a/Makefile
+++ b/Makefile
@@ -70,6 +70,9 @@
 USE_ONOS_BAZEL_OUTPUT        ?=
 USE_LOCAL_SNAPSHOT_ARTIFACTS ?=
 KARAF_VERSION                := 4.2.14
+# We use proprietary tools such as Intel P4 Studio for which we are not allowed
+# to make the output public in CI jobs.
+export SHOW_SENSITIVE_OUTPUT ?= false
 
 # Trellis-Control related
 export TRELLIS_CONTROL_ROOT  := $(shell pwd)/trellis-control