Merge launcher into retro-core

[ducalex]

I want retro-core to become a standalone esp32 application, containing the launcher and the core emulators. It should be buildable without rg_tool.py, with standard esp-idf tooling.

Benefits:

• retro-core can now be built and flashed with idf.py (assuming the user doesn't care about DOOM and such)
• retro-core being standalone means it can be built for platforms with no concept of partitions/multiple exes
• Reduced firmware size
• All retro-core apps can now access wifi if they want
• Faster builds because it means one less app

Drawbacks:

• It breaks git history for launcher files (but --follow mostly works around that)
• The presence of the wifi stack, even when not enabled, reserves some amount of internal memory

I've tried on several occasions to merge the launcher and retro-core (latest attempt here), but every time there are issues that stops me. And then I forget what the problem was.

This issue is meant to track problems found during testing.

[ducalex]

Currently the blocking problems are:

• Must investigate the wifi stack's usage of memory and see if it affects any emulator and if it can be reduced via menuconfig options