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: NSimon Glass <sjg@chromium.org>

tools/buildman/buildman

-buildman.py
\ No newline at end of file
+main.py
\ No newline at end of file