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/patman/patchstream.py b/tools/patman/patchstream.py
index 4955f6a..940b50c 100644
--- a/tools/patman/patchstream.py
+++ b/tools/patman/patchstream.py
@@ -711,7 +711,7 @@
     """
     params = gitutil.log_cmd(commit_range, reverse=True, count=count,
                             git_dir=git_dir)
-    return command.run_pipe([params], capture=True).stdout
+    return command.run_one(*params, capture=True).stdout
 
 def get_metadata_for_list(commit_range, git_dir=None, count=None,
                           series=None, allow_overwrite=False):