buildman: Rename the main module

Python does not like the module name being the same as the module
directory. To allow buildman modules to be used from other tools, rename
it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman b/tools/buildman/buildman
index e4fba2d..11a5d8e 120000
--- a/tools/buildman/buildman
+++ b/tools/buildman/buildman
@@ -1 +1 @@
-buildman.py
\ No newline at end of file
+main.py
\ No newline at end of file
diff --git a/tools/buildman/buildman.py b/tools/buildman/main.py
similarity index 100%
rename from tools/buildman/buildman.py
rename to tools/buildman/main.py