boot: zephyr: migrate CDC ACM serial recovery to USB_DEVICE_STACK_NEXT

[JayToltTech]

Summary

• Replace the legacy USB device stack with USB_DEVICE_STACK_NEXT for CDC-ACM serial recovery
• Initialize USB lazily in boot_uart_fifo_init() only when serial recovery is triggered, avoiding interference with application USB on normal boots
• Add self-contained USBD CDC ACM module (usbd_cdc_serial.c/.h) with configurable USB descriptors (VID, PID, manufacturer, product, self-powered, max power) via new Kconfig options
• Feed the watchdog while waiting for USB host connection to prevent timeout before the serial recovery loop is reached
• Enlarge nrf52840 boot partitions to accommodate the larger USB stack
• Remove stale legacy USB Kconfig options from board configs
• Add flash-saving options (disable BOS, vendor requests, deferred messages) to recovery conf files

Supersedes #2634 (narrowed scope to CDC-ACM only, removed unrelated logging/SWO changes).

Builds on the approach from #2346 with key improvements:

• Lazy USB init instead of CDC_ACM_SERIAL_INITIALIZE_AT_BOOT
• Watchdog feeding during USB host wait
• Dongle partition overlay for the larger USB stack
• Direct context accessor instead of fragile STRUCT_SECTION_GET index

Test plan

• Build nrf52840dk with usb_cdc_acm_log_recovery.conf (92.49% flash)
• Build nrf52840dongle with usb_cdc_acm_recovery.conf (99.71% flash)
• Functional test: enter serial recovery, upload image via mcumgr over USB CDC-ACM
• Verify normal boot path does not initialize USB

[Copilot]

Pull request overview

This pull request migrates CDC ACM serial recovery from the legacy USB device stack to the new USB_DEVICE_STACK_NEXT in MCUboot. The key improvement is lazy USB initialization - USB is only initialized when serial recovery is actually triggered, avoiding interference with application USB on normal boots.

Changes:

• Adds self-contained USB CDC ACM module with configurable descriptors via Kconfig
• Implements lazy USB initialization in serial recovery path with watchdog feeding during USB host wait
• Enlarges nrf52840 boot partitions (64KB → 72KB) to accommodate larger USB stack
• Removes legacy USB stack configuration options from board files

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
docs/readme-zephyr.md	Documents new Kconfig options for USB descriptor configuration
boot/zephyr/usbd_cdc_serial.h	Header for new USB CDC ACM module
boot/zephyr/usbd_cdc_serial.c	Self-contained USB stack setup with lazy initialization
boot/zephyr/usb_cdc_acm_recovery.conf	Adds product string override and flash-saving options
boot/zephyr/usb_cdc_acm_log_recovery.conf	Same configuration for logging variant
boot/zephyr/serial_adapter.c	Updates to use new USB module with lazy init and watchdog feeding
boot/zephyr/main.c	Adds USB cleanup on boot with -EALREADY handling for lazy init
boot/zephyr/boards/nrf52840dongle_nrf52840_big.overlay	New overlay with enlarged boot partition for dongle
boot/zephyr/boards/nrf52840dongle_nrf52840.conf	Removes legacy USB stack config options
boot/zephyr/boards/nrf52840_big.overlay	Enlarges boot partition from 64KB to 72KB
boot/zephyr/Kconfig.serial_recovery	Adds new Kconfig options for USB descriptor customization
boot/zephyr/CMakeLists.txt	Conditionally compiles usbd_cdc_serial.c
boot/zephyr/sample.yaml	Updates test configuration to use new overlay for dongle

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[henrikbrixandersen]

Same as with #2346, we cannot merge this without also migrating the USB DFU support the device_next. Otherwise, you will have a mix of dependencies, some of the MCUBoot code depending on the legacy stack, other on the new stack.