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/toolchain.py b/tools/buildman/toolchain.py
index 958f36f..5d051e0 100644
--- a/tools/buildman/toolchain.py
+++ b/tools/buildman/toolchain.py
@@ -100,7 +100,7 @@
         else:
             self.priority = priority
         if test:
-            result = command.run_pipe([cmd], capture=True, env=env,
+            result = command.run_one(*cmd, capture=True, env=env,
                                      raise_on_error=False)
             self.ok = result.return_code == 0
             if verbose: