buildman: Detect boards with no CONFIG_TARGET defined

We generally expected exactly one of these. Add a check for it.

Note: This warning is not displayed by default. An option will be added
to enable it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/boards.py b/tools/buildman/boards.py
index 56fb6f4..dabf694 100644
--- a/tools/buildman/boards.py
+++ b/tools/buildman/boards.py
@@ -262,6 +262,10 @@
                 else:
                     target = tname
 
+        if not target:
+            cfg_name = expect_target.replace('-', '_').upper()
+            warnings.append(f'WARNING: {leaf}: No TARGET_{cfg_name} enabled')
+
         params['target'] = expect_target
 
         # fix-up for aarch64