shared/tinyusb, extmod: Add USB Network (NCM) with runtime class selection.

[andrewleech]

Summary

This adds USB networking via NCM (Network Control Model) and a runtime USB class selection API. The work is in four phases that could be split into separate PRs, though phases 1-3 are dependent on prior work so would need to stay stacked.

Background network support - a couple of small prerequisites: a weak atoi() implementation for lwIP's netif_find(), consolidating TinyUSB source includes across port Makefiles to use upstream tinyusb.mk, a send_router flag on the DHCP server so point-to-point links don't advertise a default gateway (which hijacks the host's internet route), making LWIP_IPV6 configurable on stm32, and moving mod_network_init() earlier in stm32's boot sequence so the netif is ready before USB enumeration.

NCM implementation - the actual network.USB_NET driver. It provides a USB Ethernet adapter via NCM that the host sees immediately on plug-in. The device runs a DHCP server and optionally mDNS so the host gets an IP and can reach the device without any manual config. The netif is auto-initialised at boot (before tusb_init()) because TinyUSB's NCM driver starts receiving as soon as the host sends SET_INTERFACE, which happens during enumeration before any Python code runs. NCM is auto-enabled on any port with lwIP and TinyUSB.

Runtime USB class selection - integer flag constants (BUILTIN_FLAG_CDC, BUILTIN_FLAG_MSC, BUILTIN_FLAG_NCM) that can be OR'd together and assigned to machine.USBDevice.builtin_driver in boot.py. This builds a dynamic configuration descriptor containing only the selected classes with compact interface/endpoint numbering. The PID is varied per combination so the host re-enumerates correctly. BUILTIN_DEFAULT is now CDC-only, MSC and NCM are opt-in. Without RUNTIME_DEVICE (ie. no machine.USBDevice) all compiled-in classes are exposed by default.

TinyUSB fixes - VBUS sensing fix for newer STM32F4/F7 HAL versions that have the VBDEN register bit, and a mboot build fix for TinyUSB-enabled boards.

Testing

Tested on NUCLEO_WB55 (stm32 with TinyUSB + lwIP) with both RUNTIME_DEVICE=1 and RUNTIME_DEVICE=0. NCM interface enumerates on Linux host, DHCP assigns an address, ping works at ~1.8ms RTT. Flag-mask class selection tested with CDC-only and CDC+NCM combinations.

Not tested on mimxrt, renesas-ra, samd, rp2 (build-tested only via the auto-enable logic). The descriptor patch offset tables for the dynamic builder haven't been verified against TinyUSB versions other than the current submodule.

Trade-offs and Alternatives

The descriptor builder uses hardcoded byte-offset patch tables derived from TinyUSB's TUD_*_DESCRIPTOR macro output. This is fragile across TinyUSB updates but pretty much unavoidable without TinyUSB exposing a programmatic descriptor API. The offset tables include a comment noting this dependency.

The previous usbd_net branch used an opaque C type with an internal flags byte for class selection, requiring casting in the descriptor callback. The integer flag approach here is simpler at both the Python and C level.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.46%. Comparing base (5c00edc) to head (7d1a1ed).
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Additional details and impacted files

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##           review/stm32_tinyusb_ncm      #31   +/-   ##
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  Coverage                     98.46%   98.46%           
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  Files                           176      176           
  Lines                         22784    22784           
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  Hits                          22435    22435           
  Misses                          349      349           

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[github-actions]

Code size report:

Reference:  qemu/boards/MPS2_AN500: Configure and enable ROMFS. [ed463ed]
Comparison: stm32/usbd_conf: Fix USB VBUS sensing for newer STM32F4/F7 HAL versions. [merge of 04ef70d]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
      esp32:    +0 +0.000% ESP32_GENERIC
     mimxrt:    +0 +0.000% TEENSY40
        rp2: +13596 +1.478% RPI_PICO_W[incl +10552(bss)]
       samd:  +504 +0.183% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32