util: Add maintainer tools to create upstream patches

util: Add maintainer tools to create upstream patches

This changeset adds a maintainer script, create_patches.sh, that can
be used to prepare for upstream from a git repository. The script can
be used to generate patches in Mercurial or git format. The commit
messages in the exported patches are all filtered, see
upstream_msg_filter.sed, to ensure that irrelevant meta data isn't
included in the upstream commit.

Kudos to Curtis Dunham and Nikos Nikoleris for reviews and usability
enhancements for earlier versions of this patch.

Change-Id: Ia4cd089a32834b5e046ef58c0a173ca285b77bca
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>