Build: fix silent mode with make 3.8x

With make 3.8x, in case of 'make all -s -w', MAKEFLAGS equals 'ws'
This patch correctly catches the flag 's'

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Pascal Brand <pascal.brand@st.com>
diff --git a/Makefile b/Makefile
index 718efc6..ad81a9c 100644
--- a/Makefile
+++ b/Makefile
@@ -46,7 +46,7 @@
 cmd-echo-silent := true
 endif
 else                                    # make-3.8x
-ifneq ($(filter s% -s%,$(MAKEFLAGS)),)
+ifneq ($(findstring s, $(MAKEFLAGS)),)
 cmd-echo-silent := true
 endif
 endif