releasetools: Fallback to "mkbootimg_args" if "recovery_mkbootimg_args" not found.

For older target_files.zip, since "recovery_mkbootimg_args" may not be
set fallback to the earlier config -"mkbootimg_args" for recovery image.
This will make releasetools backward compatible, and usable for
older/already generated target-files package.

Change-Id: If40359130b8e6da48cb8c345d8cb079f457fe36f