tools: Plumb in capture control

Add control of capturing output into u_boot_pylib and the tools which
use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_fdt.py b/tools/dtoc/test_fdt.py
index 0b01518..a0bed4e 100755
--- a/tools/dtoc/test_fdt.py
+++ b/tools/dtoc/test_fdt.py
@@ -969,7 +969,7 @@
     """
     test_name = names[0] if names else None
     result = test_util.run_test_suites(
-        'test_fdt', False, False, False, processes, test_name, None,
+        'test_fdt', False, False, False, False, processes, test_name, None,
         [TestFdt, TestNode, TestProp, TestFdtUtil])
 
     return (0 if result.wasSuccessful() else 1)