buildman: Make sure that -o is given with -w

It is a bad idea to use the default output directory ('..') with -w since
it does a build in that directory and writes various files these.

Require that -o is given to avoid this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/cmdline.py b/tools/buildman/cmdline.py
index 1377b9d..680c072 100644
--- a/tools/buildman/cmdline.py
+++ b/tools/buildman/cmdline.py
@@ -76,8 +76,7 @@
           default=False, help="Do a dry run (describe actions, but do nothing)")
     parser.add_option('-N', '--no-subdirs', action='store_true', dest='no_subdirs',
           default=False, help="Don't create subdirectories when building current source for a single board")
-    parser.add_option('-o', '--output-dir', type='string',
-          dest='output_dir', default='..',
+    parser.add_option('-o', '--output-dir', type='string', dest='output_dir',
           help='Directory where all builds happen and buildman has its workspace (default is ../)')
     parser.add_option('-O', '--override-toolchain', type='string',
           help="Override host toochain to use for sandbox (e.g. 'clang-7')")