serf: Document and add upstream status field for 0003-gen_def.patch

(From OE-Core rev: 60909e5d4ac94ed77699a569960bb689acb432dd)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/serf/serf/0003-gen_def.patch

@@ -3,6 +3,11 @@ From: Khem Raj <raj.khem@gmail.com>
 Date: Mon, 3 Sep 2018 11:12:27 -0700
 Subject: [PATCH] gen_def
 
+This is partial port because half of this patch is already present in our
+patches
+
+Upstream-Status: Backport [https://github.com/apache/serf/commit/8f07c9626106873a594fd07e7c257e9080d8660b]
+Signed-off-by: Khem Raj <raj.khem@gmail.com>
 ---
  SConstruct | 2 ++
  1 file changed, 2 insertions(+)