buildman: Split out Boards into its own file

Use a separate file for the Boards class so that its name matches the
module name.

Fix up the function names to match the pylint style and fix some other
warnings.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/test.py b/tools/buildman/test.py
index d7306fb..daf5467 100644
--- a/tools/buildman/test.py
+++ b/tools/buildman/test.py
@@ -10,6 +10,7 @@
 import unittest
 
 from buildman import board
+from buildman import boards
 from buildman import bsettings
 from buildman import builder
 from buildman import cfgutil
@@ -131,7 +132,7 @@
             self.commits.append(comm)
 
         # Set up boards to build
-        self.brds = board.Boards()
+        self.brds = boards.Boards()
         for brd in BOARDS:
             self.brds.add_board(board.Board(*brd))
         self.brds.select_boards([])