[border-router] add info about required flag for dongle in OpenThread Border Router Setup guide

[jrhodie]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @jrhodie, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the OpenThread Border Router Setup guide by clarifying a mandatory build flag for nRF52840 USB dongles. The update ensures users are aware of the -DOT_BOOTLOADER=USB flag, which is essential for enabling the USB DFU trigger and successfully flashing firmware onto the dongles, thereby preventing common setup issues.

Highlights

• Documentation Update: Added critical information regarding the -DOT_BOOTLOADER=USB flag requirement when building firmware for nRF52840 USB dongles, ensuring successful firmware loading for both RCP and NCP designs.

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[gemini-code-assist]

Code Review

This pull request adds an important note to the OpenThread Border Router setup guide about the required -DOT_BOOTLOADER=USB flag when building firmware for nRF52840 USB dongles. This is a helpful clarification for users. My review includes a suggestion to format this new information as a > Note: block to improve its visibility and maintain consistency with the document's existing style, while also correcting a minor spacing issue.

[abtink]

The change LGTM. That said, I am not familiar with nRF RCP and its requirements. I suggest someone from Nordic double-check this. I will add/suggest some reviewer @LuDuda .

[Vyrastas]

For context @LuDuda , we got feedback internally on openthread.io that said:

---

When building firmware for the nRF52840 USB dongles, you must use the flag -DOT_BOOTLOADER=USB in order to enable the USB DFU trigger. This must be present when building for both the RCP and NCP design. If the flag is not present, the compiled firmware can not be loaded onto the dongle.

[Vyrastas]

One minor nit

[Vyrastas]

LGTM for doc update only

[jrhodie]

@LuDuda - does the wording look ok, thanks?

[LuDuda]

Thank you for this PR!

I would slightly change the wording to indicate that the primary reason for using this flag is to select the correct memory map (linker script).

Other than this I'm accepting.

[jwhui]

Thanks! 👍🏼