buildman: Add a way to build a particular target

At present buildman only supports building the default target. Generally
this is what is wanted, but in some cases boards erroneously have a
different target for product extra files.

Add a --target option to help. Also add a comment indicating which
letters are free for new options.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index b8578d5..b4cb663 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -285,6 +285,8 @@
         """
         if config_only:
             args.append('cfg')
+        elif self.builder.build_target:
+            args.append(self.builder.build_target)
         result = self.make(commit, brd, 'build', cwd, *args, env=env)
         cmd_list.append([self.builder.gnu_make] + args)
         if (result.return_code == 2 and