Add the Skylark machinery for downloading code to a robot

The Python script it runs currently does basically nothing.

Change-Id: Ic2d5bdccc7a8bcb6ce5b8c6987ec151ccfbce3d2
diff --git a/aos/prime/downloader/downloader.py b/aos/prime/downloader/downloader.py
new file mode 100644
index 0000000..de3f872
--- /dev/null
+++ b/aos/prime/downloader/downloader.py
@@ -0,0 +1,16 @@
+# This file is run by shell scripts generated by the aos_downloader Skylark
+# macro. Everything before the first -- is a hard-coded list of files to
+# download.
+
+import sys
+import subprocess
+
+def main(argv):
+  srcs = argv[:argv.index('--')]
+  args = argv[argv.index('--') + 1:]
+
+  # TODO(Brian): Actually do something useful...
+  subprocess.check_call(['echo', 'scp'] + srcs + ['%s:/tmp' % args[0]])
+
+if __name__ == '__main__':
+  main(sys.argv)