Makefile: Explicitly silence sub-makes

GNU Make 4.3 doesn't propagate a global .SILENT to sub-processes
anymore. Let's make it explicit to maintain the behaviour we are
used to.

From the changelog:

  [SV 54740] Ensure .SILENT settings do not leak into sub-makes

Change-Id: I3de51c245d3344b062dc0fe9c62b8d5c0ac5e67d
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38931
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Makefile

@@ -82,6 +82,7 @@ Q:=@
 ifneq ($(V),1)
 ifneq ($(Q),)
 .SILENT:
+MAKEFLAGS += -s
 endif
 endif