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.
When lwIP creates a incoming connection socket of a listen socket, it
sets its recv callback to one which discards incoming data. We set
proper callback only in accept() call, when we allocate Python-level
socket where we can queue incoming data. So, in lwIP accept callback
be sure to set recv callback to one which tells lwIP to not discard
incoming data.
Upon start-up, _boot module is executed from frozen files to do early
initialization, e.g. create and mount the flash filesystem. Then
"boot.py" is executed if it exists in the filesystem. Finally, "main.py"
is executed if exists to allow start-on-boot user applications.
This allows a user to make a custom boot file or startup application
without recompiling the firmware, while letting to do early initialization
in Python code.
Based on RFC https://github.com/micropython/micropython/issues/1955.
Damien George
Paul Sokolovsky