opkg: Fix build reproducibility issue

A build date was leaking into the generated docs and makefile used for
ptests leading to reproducibility issues each time the month changed.

Add a patch to use SOURCE_DATE_EPOCH to derive it if available.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6a9ca7ae)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>