patman: Support using a particular SMTP server

Some environments require providing the '--smtp-server' argument to
'git send-email'. Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index 606780e..7917fc8 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -107,6 +107,7 @@
 ignore_errors: True
 process_tags: False
 verbose: True
+smtp_server: /path/to/sendmail
 
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