buildman: Fix problem with non-existent output directories

Now that we have buildman telling genboards.cfg to use an output
directory we need to ensure that it exists.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Fixes: bc750bca1246 ("tools: buildman: Honor output directory when generating boards.cfg")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index 216012d..c55a65d 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -201,6 +201,8 @@
 
     # Work out what subset of the boards we are building
     if not boards:
+        if not os.path.exists(options.output_dir):
+            os.makedirs(options.output_dir)
         board_file = os.path.join(options.output_dir, 'boards.cfg')
         genboardscfg = os.path.join(options.git, 'tools/genboardscfg.py')
         status = subprocess.call([genboardscfg, '-o', board_file])