b4-config: configure b4 for U-Boot

`b4` is a commandline tool to make patch-based development easier.

Provide a .b4-config file to match U-Boot's development preference about
who is cc'd on patch submission.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
[trini: Reword slightly]

.b4-config

@@ -0,0 +1,7 @@
+# Configuration for the `b4` tool
+# See https://b4.docs.kernel.org/en/latest/config.html
+
+[b4]
+    send-series-to = u-boot@lists.denx.de
+    send-auto-to-cmd = echo ""
+    send-auto-cc-cmd = scripts/get_maintainer.pl