Add basic b4 config file

b4[1] is a convenient tool for mail-based contribution. A config file[2]
tracked in the tree lets the project ship a few defaults that match the Xen
patch submission rules.

This aims to help new contributors who have never used a mailing list avoid
mistakes when sending their patch series.

The tree has no scripts/checkpatch.pl or equivalent, so there is nothing
for b4 to run per patch. Disable the needs-checking pre-flight check so a
series can be sent without first running b4 prep --check.

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

Signed-off-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

.b4-config

@@ -0,0 +1,10 @@
+#
+# Common b4 settings for sending patches to the Xen Project upstream.
+# https://b4.docs.kernel.org/
+#
+
+[b4]
+	send-series-to = xen-devel@lists.xenproject.org
+	send-auto-to-cmd = echo
+	send-auto-cc-cmd = ./scripts/get_maintainer.pl
+	midmask = https://lore.kernel.org/xen-devel/%s