buildman: Add a test for --boards

Add a simple functional test for the --boards option. Fix the example in
the docs while we are here. Also improve the docs for Builder.count so it
is clearer what it contains.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builder.py

@@ -163,7 +163,8 @@ class Builder:
         checkout: True to check out source, False to skip that step.
             This is used for testing.
         col: terminal.Color() object
-        count: Number of commits to build
+        count: Total number of commits to build, which is the number of commits
+            multiplied by the number of boards
         do_make: Method to call to invoke Make
         fail: Number of builds that failed due to error
         force_build: Force building even if a build already exists

tools/buildman/buildman.rst

@@ -159,7 +159,7 @@ on the command line:
 
 .. code-block:: bash
 
-  buildman --boards sandbox,snow --boards
+  buildman --boards sandbox,snow --boards firefly-rk3399
 
 It is convenient to use the -n option to see what will be built based on
 the subset given. Use -v as well to get an actual list of boards.

tools/buildman/func_test.py

@@ -1047,3 +1047,14 @@ endif
         self._RunControl('-x', 'board2', '-x' 'board4')
         self.assertEqual(['board0', 'board1'],
                          [b.target for b in self._boards.get_selected()])
+
+    def test_single_boards(self):
+        """Test building single boards"""
+        self._RunControl('--boards', 'board1')
+        self.assertEqual(1, self._builder.count)
+
+        self._RunControl('--boards', 'board1', '--boards', 'board2')
+        self.assertEqual(2, self._builder.count)
+
+        self._RunControl('--boards', 'board1,board2', '--boards', 'board4')
+        self.assertEqual(3, self._builder.count)