build-sys: silence make by default or V=0

Move generic make flags in MAKEFLAGS (SUBDIR_MAKEFLAGS is more qemu specific).

Use --quiet to silence make 'is up to date' message.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180104160523.22995-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>