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

[andrewleech]

Summary

This is a rewrite of micropython#16459 on a current master base. The STM32 TinyUSB enablement commits from that branch have since landed, so this one focuses on the NCM networking layer and the runtime USB class selection mechanism.

The core addition is network.USB_NET - an NCM network interface that lets a host computer see the MicroPython device as a USB Ethernet adapter. The device runs a DHCP server and optionally mDNS, so the host gets an IP and can reach the device immediately after plug-in. The driver integrates with lwIP via the standard netif interface.

USB class selection is handled through 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. A dynamic configuration descriptor is built at runtime 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.

Port Makefiles for alif, mimxrt, renesas-ra, and samd are refactored to use TinyUSB's upstream tinyusb.mk instead of hand-maintained file lists. NCM is auto-enabled on any port with lwIP and TinyUSB.

Testing

Built for NUCLEO_F429ZI (stm32 with TinyUSB + lwIP). NCM interface enumerates on Linux and macOS hosts. DHCP assigns an address and the device is reachable over USB. Flag-mask class selection tested with CDC-only, CDC+NCM, and CDC+MSC+NCM combinations - host sees distinct devices and re-enumerates on each change.

Not tested: mimxrt, renesas-ra, samd, rp2 (build-tested only via the auto-enable logic). The descriptor patch offset tables for the dynamic builder have not 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 macro-generated descriptors. This is fragile across TinyUSB updates but 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 hacks 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 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:  tools/ci.sh: Increase qemu_arm test run timeout. [5c00edc]
Comparison: stm32: Fix mboot build with TinyUSB-enabled boards. [merge of 7d1a1ed]
  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: +13668 +1.485% RPI_PICO_W[incl +10552(bss)]
       samd:  +504 +0.183% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32