Signed-off-by: Damien George <damien@micropython.org>

This commit implements basic NVS support for the esp32.  It follows the
pattern of the esp32.Partition class and exposes an NVS object per NVS
namespace.  The initial support provided is only for signed 32-bit integers
and binary blobs.  It's easy (albeit a bit tedious) to add support for
more types.

See discussions in: #4436, #4707, #6780

This commit fixes the behaviour of socket.getaddrinfo on the ESP32 so it
raises an OSError when the name resolution fails instead of returning a []
or a resolution for 0.0.0.0.

Tests are added (generic and ESP32-specific) to verify behaviour consistent
with CPython, modulo the different types of exceptions per MicroPython
documentation.

This commit adds several small items to improve the support for OTA
updates on an esp32:

- a partition table for 4MB flash modules that has two OTA partitions ready
  to go to do updates
- a GENERIC_OTA board that uses that partition table and that enables
  automatic roll-back in the bootloader
- a new esp32.Partition.mark_app_valid_cancel_rollback() class-method to
  signal that the boot is successful and should not be rolled back at the
  next reset
- an automated test for doing an OTA update
- documentation updates

This commit consolidates a number of check_esp_err functions that check
whether an ESP-IDF return code is OK and raises an exception if not.  The
exception raised is an OSError with the error code as the first argument
(negative if it's ESP-IDF specific) and the ESP-IDF error string as the
second argument.

This commit also fixes esp32.Partition.set_boot to use check_esp_err, and
uses that function for a unit test.

This commit adds an idf_heap_info(capabilities) method to the esp32 module
which returns info about the ESP-IDF heaps.  It's useful to get a bit of a
picture of what's going on when code fails because ESP-IDF can't allocate
memory anymore.  Includes documentation and a test.