Addressed lintian/shellcheck warnings

[tuto193]

What:

• Adjusted all .sh/.bash scripts so they are now POSIX compliant (used shellcheck to find/fix issues).

Why:

• Platforms/GOS -Dev team was looking to get rid of hopefully all warnings/notifications (instead of just ignoring them like done until now), to ensure that the code remains stabke, compliant and compatible, and keep a clean build system, to more easily tackle future errors and problems that may arise.

How did you test it: All styling/code changes were tested locally and individually to ensure that functionality remains equal, while making mostly just superficial changes.

Checklist:

• [x] Tests
• [x] CHANGELOG Entry