Adds optional --rebuild_recovery flag to merge_target_files.

Also makes AddSystem check that an output_zip exists before attempting
to add the recovery patch to the output zip.

Bug: 128838154
Test: Running merge_target_files with --rebuild_recovery and verifying
it passes --rebuild_recovery to add_img_to_target_files.
Change-Id: I19347b2c0dabf29b7196045b18551b5d0687df2c