Should allow to diagnose/try to recover FS easier.

That apparently will only help folks who read the docs on how to disable,
but could use a quick reminder straight in boot.py. For the developers,
it's important to have debug logging enabled in development branch
(master).

Leads to less fragmentation at teh time user code starts.

It interferes with running testsuite. master branch should be optimized for
development, so any features which interfere with that, would need to be
disabled by default.

Currently it pre-imports webrepl, but doesn't start it.

Main entry point is _boot.py which checks whether FAT FS in flash mountable,
and if so, mounts it. Otherwise, it checks if flash is empty, and if so,
performs initial module setup: makes FAT FS, configures default AP name,
etc. As a last option, if flash is not empty, and could not be mounted,
it means filesystem corruption, and warning message with instructions is
printed in an infinite loop.