add basic b4 config file

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 set the To recipients
to always add, in our case the mailing list.

This shouldn't be necessary if we had a script that b4 prep --auto-to-cc
could call to find the mail address(es) to send to. While we could do
that, this opens the door to people trying to add a way to notify some
people (e.g. a MAINTAINERS file) which is not somewhere the project
wants to go right now, and this also makes sure the address is always
added even if one forgets to run b4 prep --auto-to-cc (which is part of
the typical workflow for U-Boot and Linux kernel, but is not necessary
thanks to this patch).

Additionally, automatically provide a default prefix so that one doesn't
need to not forget to run `b4 prep --set-prefixes meta-rockchip` for
each series.

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

Reviewed-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>

.b4-config

@@ -0,0 +1,3 @@
+[b4]
+  send-series-to = yocto-patches@lists.yoctoproject.org
+  send-prefixes = meta-rockchip