Refactor build script for enhanced readability and efficiency

[tsvenbla]

• Modularize script using functions for downloading files and building modules.
• Introduce configurable variables for FTP details, kernel source directory, and build directory at the top of the script for easy modification.
• Utilize an array to list and manage multiple files, streamlining the download process.
• Improve directory management by creating and cleaning up build directories within functions, ensuring a cleaner build environment.
• Enhance error handling by adding exit conditions on failure in download and upload processes.
• Implement best practices for script readability and maintainability.

Note that I have not tested this commit, I suggest doing so before any merge.