* USB: enable DFU interface and stub

* nano_nora: add Arduino Nano ESP32 board support

* [pin_remap 1/3] platform: define ARDUINO_CORE_BUILD when building core files

* [pin_remap 2/3] core,libs: add pin remap hooks

* [pin_remap 3/3] nano_nora: implement and enable pin remap

* nano_nora: fix: reset all matrix connections at boot

* nano_nora: add recovery image for release/v2.x

* nano_nora: use official Arduino branding

* nano_nora: core split + recovery mode rework

Use an absolute address in SPIRAM to store the magic tokens, almost at the
end of the memory, to avoid the markers from being overwritten on any kind
of sketch and core combination.

Also, only start the recovery once if a valid binary is present in the
Flash, by immediately setting that for the next boot when recovery
starts.

* platform: fix: use {compiler.sdk.path} for sdk path

In preparation for the sdk -> tool transition

* package_index: remove dfu-util from template

The tool is already available in mainline package_index.json

* on_release: allow single board packages

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Co-authored-by: Luca Burelli <l.burelli@arduino.cc>