micropython: use ESP-IDF VFS layer for MPy VFS

Previously MicroPython would mount LittleFS directly from the partition
by itself, using its own VFS stack.

This reconfigured the VFS layer in MicroPython to use POSIX
open/read/write/etc calls, which are backed by ESP-IDF. Now, instead of
managing (possibly multuple) block devices at different mountpoints,
MicroPython simply passes through all VFS access to the ESP-IDF VFS.

Then, we make the ESP-IDF VFS mount a wear-leveled FAT (backed by the
same partition which previously held LittleFS) onto /.