Makefiles: Hide skipping submodule info unless V=1

Currently, git prints out the submodules that are being skipped twice
on many builds.  This patch hides that output unless the build is set
to show it with `make V=1`.  This is the normal way of showing the extra
information during the build.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I7b5c7f1f79dcc88793a9a21f2e92e7accc5de1e0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59511
Reviewed-by: Patrick Georgi <patrick@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Makefile

@@ -63,6 +63,7 @@ ifneq ($(V),1)
 ifneq ($(Q),)
 .SILENT:
 MAKEFLAGS += -s
+quiet_errors := 2>/dev/null
 endif
 endif