feat(dfu_ext): Add ext dfu flash

[Schievel1]

adds possibilty to use external flash as dfu partition

Also adds noswap feature, to omit DFU and directly flash the firmware to ACTIVE partition.
And defmt feature for logs.

[Copilot]

Pull request overview

This PR extends the bootloader to support using an external SPI NOR flash (W25Q-class) as the DFU download partition (dfu_ext), alongside adding a noswap build mode and broadening nRF support to include nRF52833 (via a shared nrf528xx feature). It also introduces optional defmt RTT logging and updates the build/release tooling and documentation to cover the new variants.

Changes:

• Add dfu_ext flow: external SPI flash is used as the DFU partition for swap-based updates (RP2040 + nRF52) and for USB DFU (nRF52).
• Add noswap flow: disable swap logic and boot ACTIVE directly, with updated build variants and docs.
• Add nRF52833 support and unify nRF gating under nrf528xx; update build scripts, release workflow, and README accordingly.

Reviewed changes

Copilot reviewed 31 out of 50 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/rp2040.rs	Adds dfu_ext external-flash DFU partition support and refactors swap/boot flow gating.
src/nrf52840.rs	Unifies nRF boot flow under nrf528xx and adds dfu_ext external-flash DFU partition + USB DFU entry handling.
src/main.rs	Adds feature exclusivity checks, shared SWAP_PAGE_SIZE, optional defmt startup logging, and makes current_progress mutex-generic.
src/led_pwm.rs	Renames nRF feature gating from nrf52840 to nrf528xx.
src/driver/w25q.rs	New W25Q-compatible SPI NOR flash driver implementing embedded_storage NOR flash traits.
src/driver/mod.rs	New driver module export for the W25Q flash implementation.
src/dfu.rs	Adds run_dfu_usb_ext for writing DFU images to external SPI flash (nRF52), plus defmt logging hooks.
rmk-boot-rp2040-8mb.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 8MB variant.
rmk-boot-rp2040-8mb-noswap.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 8MB noswap variant.
rmk-boot-rp2040-4mb.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 4MB variant.
rmk-boot-rp2040-4mb-noswap.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 4MB noswap variant.
rmk-boot-rp2040-2mb.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 2MB variant.
rmk-boot-rp2040-2mb-noswap.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 2MB noswap variant.
rmk-boot-rp2040-16mb-noswap.hex	Adds/updates prebuilt Intel HEX artifact for RP2040 16MB noswap variant.
Readme.org	Updates docs for nRF52833, noswap, dfu_ext, build targets, LED patterns, and SPI pin defaults.
Makefile.toml	Expands cargo-make tasks to cover nRF52833, noswap, and dfu_ext UF2/HEX build matrices; renames RP2040 tasks.
Cargo.toml	Adds nrf528xx feature factoring, nrf52833, noswap, dfu_ext, and optional defmt dependencies/features.
Cargo.lock	Updates lockfile for new dependencies/features (notably defmt and related transitive deps).
build.rs	Extends layout generation for noswap/dfu_ext, adds nRF52833 RAM sizing and variant slugging, and adds defmt link arg.
.github/workflows/release.yml	Builds and uploads additional nRF52833 and noswap ELF assets; broadens uploaded asset patterns.
.cargo/config.toml	Sets DEFMT_LOG=trace environment variable for builds.

Suppressed comments (1)

src/nrf52840.rs:120

• Same issue as above: ext_mutex_data is stack-local, but ext_mutex is created as &'static via addr_of!, which is undefined behavior. This also means the DFU partition backing store may alias invalid memory during bootloader swap operations.

Please move the external-flash mutex into static storage (e.g. static mut MaybeUninit<...> + one-time init) and reference that instead of extending a stack reference to 'static.

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