Refactor PlatformIO build scripts

[valeros]

This pull request includes the following two major improvements:

• Refactored structure of the build scripts. Currently, the build scripts for each ESP32 family include a lot of duplicated code. Updating this code in four different places is tedious and error-prone. In this PR, the common code is moved to the generic platformio-build.py script.
• Updated process of searching for a proper partition table and bootloader image. It now behaves similarly to the Arduino IDE by searching in the variant directory in the first place.

I believe this PR shouldn't affect the autogeneration process implemented in the esp32-arduino-lib-builder script as only the common parts of the code were moved.

[me-no-dev]

merged! Will apply this to the lib-builder and see the result later :)