tools: buildman: Don't use the working dir as build dir

When the U-Boot base directory happens to have the same name as the branch
that buildman is directed to use via the '-b' option and no output
directory is specified with '-o', buildman happily starts removing the
whole U-Boot sources eventually only stopped with the error message:

OSError: [Errno 20] Not a directory: '../<branch-name>/boards.cfg

Add a check to avoid this and also deal with the case where '-o' points
to the source directory, or any subdirectory of it.

Finally, tidy up the confusing logic for removing the old tree when using
-b. This is only done when building a branch.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
diff --git a/tools/buildman/func_test.py b/tools/buildman/func_test.py
index 8d96c1a..9206fb2 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -519,3 +519,12 @@
         self._RunControl('-b', self._test_branch, clean_dir=False)
         self.assertEqual(self._builder.count, self._total_builds)
         self.assertEqual(self._builder.fail, 0)
+
+    def testBadOutputDir(self):
+        """Test building with an output dir the same as out current dir"""
+        self._test_branch = '/__dev/__testbranch'
+        with self.assertRaises(SystemExit):
+            self._RunControl('-b', self._test_branch, '-o', os.getcwd())
+        with self.assertRaises(SystemExit):
+            self._RunControl('-b', self._test_branch, '-o',
+                             os.path.join(os.getcwd(), 'test'))