Default to no target prefix when generating Android build files.

Rather than `None` :)

Change-Id: I8d7429746abb51c96b70b2e4cca21c0abe9cc40c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/78227
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 5d86f5c..7c5b181 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -677,7 +677,7 @@
       '|'.join(sorted(ALL_PLATFORMS.keys())))
   parser.add_option('--prefix', dest='prefix',
       help='For Bazel, prepend argument to all source files')
-  parser.add_option('--target-prefix', dest='target_prefix',
+  parser.add_option('--target-prefix', dest='target_prefix', default='',
       help='For Android, prepend argument to all target names')
   options, args = parser.parse_args(sys.argv[1:])
   PREFIX = options.prefix