st3m: move some main-specific stuff to main

We attempt to share st3m with the recovery firmware, and thus must
eliminate two components that can't live in the recovery firmware:

1. The startup function, which contains references to all matters of
   subfunctionality we don't want (audio, micropython, etc)
2. The FS code, which contains references to the tarball extraction
   logic (which we don't want to embed into recovery mode either).