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/func_test.py b/tools/buildman/func_test.py
index b45eb95..d70849b 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -1152,3 +1152,13 @@
             'board': 'ARM Board 0',
             'config': 'config0',
             'target': 'board0'}, []), res)
+
+    def testTarget(self):
+        """Test that the --target flag works"""
+        lines = self.check_command('--target', 'u-boot.dtb')[0]
+
+        # It should not affect the defconfig line
+        self.assertNotIn(b'u-boot.dtb', lines[0])
+
+        # It should appear at the end of the build line
+        self.assertEqual(b'u-boot.dtb', lines[1].split()[-1])