u_boot_pylib: Add a function to run a single command

Add a helper to avoid needing to use a list within a list for this
simple case.

Update existing users of runpipe() to use this where possible.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/func_test.py b/tools/buildman/func_test.py
index 51a2366..1afc2fb 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -232,8 +232,8 @@
         self._toolchains.Add('gcc', test=False)
 
     def _RunBuildman(self, *args):
-        return command.run_pipe([[self._buildman_pathname] + list(args)],
-                capture=True, capture_stderr=True)
+        all_args = [self._buildman_pathname] + list(args)
+        return command.run_one(*all_args, capture=True, capture_stderr=True)
 
     def _RunControl(self, *args, brds=False, clean_dir=False,
                     test_thread_exceptions=False, get_builder=True):
@@ -445,7 +445,7 @@
             stage: Stage that we are at (mrproper, config, build)
             cwd: Directory where make should be run
             args: Arguments to pass to make
-            kwargs: Arguments to pass to command.run_pipe()
+            kwargs: Arguments to pass to command.run_one()
         """
         self._make_calls += 1
         out_dir = ''