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
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