Robustify setting silent mode

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Makerules

@@ -57,7 +57,7 @@ SHELL_SET_X := set +x
 define rel_srcdir
 	$(shell $(CONFIG_SHELL) $(top_srcdir)/extra/scripts/relative_path.sh $(@D) .)
 endef
-ifneq ($(findstring s,$(MAKEFLAGS)),)
+ifneq ($(findstring -s,$(MAKEFLAGS)),)
 export MAKE_IS_SILENT := y
 SECHO := -@false
 DISP := sil