Refactor `app_gen.py` to be runnable from top level proj.

[shawnkoon]

Problem

I have to navigate into tools folder to run app_gen.py. But if I want to run anything else, I have to navigate back up to root level.

Solution

It would be nice to be able to run app_gen.py from root level (Ideally in Makefile) or by making tools module. So I can run every commands without having to switch directory back and forth.

Notes

File creation logic in app_gen.py needs to refactored as it relies on path variable.