Add support for extra scripts

[mlradu]

Pull Request Description

Add stage for extra scripts, support for running custom scripts and an example file. Add variable to config file. Update README. Rename 07.export-stage to 08.export-stage.

PR Type

• [ ] Bug fix (change that fixes an issue)
• [x] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [x] I have performed a self-review of the changes
• [x] I have commented my code, at least hard-to-understand parts
• [x] I have built Kuiper Linux image with the changes
• [ ] I have tested new image in hardware, on relevant boards
• [x] I have signed off all commits from this PR
• [x] I have updated the documentation (wiki pages, ReadMe etc)

[mlradu]

V2:

• reformulate comments in README
• add titles for 'Guidelines' and 'Examples' in the extra-script-example.sh
• example on how to source the config file in extra-script-example.sh
• delete one example for copying a file