b4-config: Add basic support for b4 contribution workflow

b4[1] is a very nice tool for mail-based contribution. A config[2] file
exists to set up a few defaults. We can use it to provide the Cc and To
recipient list for the series.

This uses the b4-wrapper-poky.py script for checking that each patch in
the series is only for one project. Indeed, poky is actually a
"collection" of multiple repositories, namely BitBake, OpenEmbedded-Core
and the Yocto Docs. One patch should therefore not make changes in
multiple of those projects otherwise it cannot be merged.

Additionally, a check is added to make sure that a series only touches
files from one project to avoid having to figure out which patch is to
be merged by which maintainer in which project repo.

Moreover, it is not uncommon to have people develop patches for those
projects from within poky. This wrapper figures out which mailing lists
to send patches to based on the files that are modified in the series.
Considering that patches to the bitbake/doc/ directory need to be sent
to both the bitbake and yocto-docs mailing list (To recipient for the
former, Cc recipient for the latter) this wrapper handles that.
A limitation of the script (lsdiff actually) is that it doesn't know how
to handle empty files, but those should be of rather rare occurrences.

Note that this script requires hardcoding of paths that are handled by
different projects to map files to projects. Anything not mapped is
assumed part of OE-Core.

[1] https://pypi.org/project/b4/
[2] https://b4.docs.kernel.org/en/latest/config.html

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

.b4-config

@@ -0,0 +1,4 @@
+[b4]
+  prep-perpatch-check-cmd = ./scripts/b4-wrapper-poky.py prep-perpatch-check-cmd
+  send-auto-cc-cmd = ./scripts/b4-wrapper-poky.py send-auto-cc-cmd
+  send-auto-to-cmd = ./scripts/b4-wrapper-poky.py send-auto-to-cmd