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/control.py b/tools/buildman/control.py
index 5109b1c..4c94891 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -785,6 +785,9 @@
                      args.verbose)
         return 0
 
+    if args.config_only and args.target:
+        raise ValueError('Cannot use --config-only with --target')
+
     # Create a new builder with the selected args
     builder = Builder(toolchains, output_dir, git_dir,
             args.threads, args.jobs, checkout=True,
@@ -810,7 +813,7 @@
             force_build_failures = args.force_build_failures,
             force_reconfig = args.force_reconfig, in_tree = args.in_tree,
             force_config_on_failure=not args.quick, make_func=make_func,
-            dtc_skip=args.dtc_skip)
+            dtc_skip=args.dtc_skip, build_target=args.target)
 
     TEST_BUILDER = builder