build: add support for make -s

Adds support for make -s to get a really silent build. This is
useful to get a shorter travis build log.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index e59d163..718efc6 100644
--- a/Makefile
+++ b/Makefile
@@ -34,18 +34,31 @@
 ifneq ($V,1)
 q := @
 cmd-echo := true
+cmd-echo-silent := echo
 else
 q :=
 cmd-echo := echo
+cmd-echo-silent := true
 endif
 
+ifneq ($(filter 4.%,$(MAKE_VERSION)),)  # make-4
+ifneq ($(filter %s ,$(firstword x$(MAKEFLAGS))),)
+cmd-echo-silent := true
+endif
+else                                    # make-3.8x
+ifneq ($(filter s% -s%,$(MAKEFLAGS)),)
+cmd-echo-silent := true
+endif
+endif
+
+
 include core/core.mk
 
 include ta/ta.mk
 
 .PHONY: clean
 clean:
-	@echo '  CLEAN   .'
+	@$(cmd-echo-silent) '  CLEAN   .'
 	${q}rm -f $(cleanfiles)
 
 .PHONY: cscope