[cvaldess]

Adds the Nordic nRF54LM20-DK as a DIY board (PRIVATE_HW, board_level = extra, like the nRF54L15-DK), with the nRF54LM20's native USB: console, serial API and clients over USB, and firmware updates over USB (serial DFU and a UF2 drive) or BLE.

Draft: it builds against packages that are not released yet (see Dependencies). It is here so the variant can be reviewed alongside them; it will be marked ready once they land.

What it adds

• variants/nrf54l15/nrf54lm20dk and boards/nrf54lm20dk.json: nRF54LM20-DK (nRF54LM20B, built as the A) with an EBYTE E22-900M30S on P3 (SPIM22), I²C on TWIM30 (P0.03/P0.04), Serial2 on UARTE21. The pin choices and the board's constraints (clock pins, port-bound serial instances, VDD) are documented in variant.h.
• USB: Serial is a CDC port on the nRF USB connector (J3), through the core's TinyUSB port for the USBHS block (USE_TINYUSB). extra_scripts/nrf54_tinyusb.py (pre) adds the TinyUSB Arduino include path, as nRF52's platform builder does itself; a -I${platformio.packages_dir}/... build flag does not work on Windows, where the path expands with backslashes that the flag parser takes as escapes.
• Updates over USB: upload_protocol = nrfutil touches the application's port at 1200 bps and uploads to the bootloader's port. The build also writes firmware-*.uf2 (extra_scripts/nrf54lm20_uf2.py, from userfirmware.hex) for the bootloader's UF2 drive. hwids keep the port finder off the on-board J-Link's VCOM ports.
• The bootloader carries the USB stack and takes 64 KB, so the application links at 0x10000 (nrf54lm20a_s145_v10_bl64k.ld).
• enterDfuMode() on nRF54: with USE_TINYUSB it calls enterUf2Dfu(), so "enter DFU" (meshtastic --enter-dfu) brings up the UF2 drive, as on nRF52. Boards without USB keep serial DFU.

Dependencies

• platform: andyshinn/platform-nordicnrf54 ashinn/nrf54lm20a (pinned), the only platform with nRF54LM20A support so far.
• Arduino core: pinned to a commit of my fork that is exactly Add the nRF54LM20-DK variant and fix the SPIM prescaler on 16 MHz instances andyshinn/nRF54_Arduino#1 (variant, SPIM prescaler fix) + mesh.options and mesh.proto are symlinks #2 (TinyUSB on USBHS, 64 KB linker script, enterUf2Dfu()).
• Bootloader: tag v0.5.0-lm20dk.1 of my fork, Add the nRF54LM20-DK with DFU over USB (CDC and UF2) andyshinn/nRF54_Bootloader#2 (USB CDC DFU and UF2) plus release/nrf54lm20dk_bootloader.hex, which the builder needs for the DFU .zip and the merged .hex.

Once those are released, platform_packages goes and platform points at the release.

Tested

On an nRF54LM20-DK with this variant (plus bench sensors not included here):

• radio (E22, EU_868), BLE with the iOS app, Telemetry over I²C;
• USB: the CDC port enumerates once at 0.7 s with the whole boot log; Meshtastic CLI and web client over USB, BLE at the same time;
• pio run -t upload over USB (1200 bps touch → bootloader → adafruit-nrfutil), the UF2 drive (copy, CURRENT.UF2, --enter-dfu) and BLE OTA. After the USB and UF2 updates the RRAM read back over SWD matched the image; after every update --info reported the new version, with the node's settings kept.

This branch, as is, builds from a clean PlatformIO core with the packages fetched from git (Windows), and that build was uploaded to the DK with pio run -t upload over USB: it boots (USB up after the SoftDevice, as intended), and --info over USB reports it. Not tested: Linux and macOS hosts, long soak.

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