kbuild: teach mkmakfile to be silent

With this fix a "make -s" is now really silent
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mkmakefile

@@ -17,7 +17,9 @@ if test -e $2/Makefile && ! grep -q Automatically $2/Makefile
 then
 	exit 0
 fi
-echo "  GEN     $2/Makefile"
+if [ "${quiet}" != "silent_" ]; then
+	echo "  GEN     $2/Makefile"
+fi
 
 cat << EOF > $2/Makefile
 # Automatically generated by $0: don't edit