Improvements for CRC and Path Handling

[MarkMuth]

Hi,

your script helped me to work with some old Huawei Update images. Thanks a lot. But since I had some issues I made the following changes:

• Make sure CRC works with Python 2.7 on Ubuntu 20.04.
• Add command line arguments for output and to skip CRC.
• Do not require to run splituapp from the current directory to allow symlinks.