buildsys: fix detection of silent make

The MAKEFLAGS variable contains all the flags without the leading dash.
From the GNU make manual:
	This variable is set up automatically by make to contain the flag
	letters that make received. Thus, if you do ‘make -ks’ then MAKEFLAGS
	gets the value ‘ks’.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

Makerules

@@ -58,7 +58,7 @@ 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