Build build_image and build_super_image as modules.

Bug: 63866463
Test: TreeHugger
Test: `atest releasetools_test`
Test: `atest releasetools_py3_test`
Change-Id: I2059a4ced709d1b2ee331a9aaaa5ca30db4ebf6b
diff --git a/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index ee05dd5..af508fe 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -18,7 +18,7 @@
 Builds output_image from the given input_directory, properties_file,
 and writes the image to target_output_directory.
 
-Usage:  build_image.py input_directory properties_file output_image \\
+Usage:  build_image input_directory properties_file output_image \\
             target_output_directory
 """