buildman: Add an option to specify the buildman config file

Add a new --config-file option (-G) to specify a different configuration
file from the default ~/.buildman.

Reported-by: Tom Rini <trini@ti.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.py b/tools/buildman/buildman.py
index f32557f..6ca8dc6 100755
--- a/tools/buildman/buildman.py
+++ b/tools/buildman/buildman.py
@@ -83,6 +83,8 @@
        help='Force build of previously-failed build')
 parser.add_option('-g', '--git', type='string',
        help='Git repo containing branch to build', default='.')
+parser.add_option('-G', '--config-file', type='string',
+       help='Path to buildman config file', default='')
 parser.add_option('-H', '--full-help', action='store_true', dest='full_help',
        default=False, help='Display the README file')
 parser.add_option('-i', '--in-tree', dest='in_tree',