feat: add Waveshare RP2040-LoRa board support

First RP2040 board and third MCU family (after ESP32-S3 and nRF52840).

Hardware: RP2040 + SX1262 radio, 850-930 MHz HF, PE4259 antenna switch,
LED on GPIO25, no OLED, USB-C via FPC ribbon cable to adapter board.

New files:
- src/hal/rp2040.rs: MCU primitives (SPI bus, USB driver, flash unique ID MAC)
- src/board/waveshare_rp2040_lora.rs: board implementation
- ld/rp2040-memory.x: linker memory layout

Also fixes cortex-m-rt/set-vtor (moved from global dep to per-board feature,
since Cortex-M0+ has no VTOR register) and makes build.rs memory.x selection
target-aware.

src/board/PORTING.md

@@ -7,6 +7,7 @@
 Create `src/board/your_board.rs`. Use an existing board as a template:
 - ESP32-S3 boards: copy `heltec_v4.rs` (uses shared helpers from `board/esp32s3.rs` and `hal/esp32s3.rs`)
 - nRF52840 boards: copy `rak_wisblock_4631.rs` (uses shared helpers from `hal/nrf52840.rs`)
+- RP2040 boards: copy `waveshare_rp2040_lora.rs` (uses shared helpers from `hal/rp2040.rs`)
 - New MCU family: create a new `hal/<mcu>.rs` with MCU primitives first
 
 `build.rs` auto-discovers your board file — any `.rs` file in `src/board/` that

src/board/waveshare_rp2040_lora.rs

@@ -0,0 +1,141 @@
+use embassy_embedded_hal::shared_bus::asynch::spi::SpiDevice;
+use embassy_rp::gpio::{Input, Level, Output, Pull};
+use embassy_rp::spi;
+use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+use embassy_time::Delay;
+
+use lora_phy::iv::GenericSx126xInterfaceVariant;
+use lora_phy::sx126x::{self, Sx1262, Sx126x};
+
+use super::traits::{BoardParts, LoRaBoard};
+use crate::hal::rp2040 as mcu;
+
+// ── Concrete peripheral types ──────────────────────────────────────
+
+type Nss = Output<'static>;
+type Iv = GenericSx126xInterfaceVariant<Output<'static>, Input<'static>>;
+type RadioSpiDevice = SpiDevice<'static, NoopRawMutex, mcu::SpiBus, Nss>;
+pub type RadioDriver = Sx126x<RadioSpiDevice, Iv, Sx1262>;
+pub type UsbDriver = mcu::UsbRpDriver;
+pub type LedDriver = crate::driver::simple_led::SimpleLed<Output<'static>>;
+
+// Display types — never instantiated (no OLED on this board), but the
+// trait requires concrete associated types that satisfy the bounds.
+pub type DisplayI2c = embassy_rp::i2c::I2c<'static, embassy_rp::peripherals::I2C0, embassy_rp::i2c::Async>;
+pub type DisplayDriver = ssd1306::Ssd1306Async<
+    ssd1306::prelude::I2CInterface<DisplayI2c>,
+    ssd1306::size::DisplaySize128x64,
+    ssd1306::mode::BufferedGraphicsModeAsync<ssd1306::size::DisplaySize128x64>,
+>;
+
+// ── Peripheral bundles ─────────────────────────────────────────────
+
+pub struct RadioParts {
+    pub driver: RadioDriver,
+    pub delay: Delay,
+}
+
+pub struct UsbParts {
+    pub driver: UsbDriver,
+}
+
+pub struct DisplayParts {
+    pub i2c: DisplayI2c,
+}
+
+// ── Display init ───────────────────────────────────────────────────
+
+pub async fn create_display(_i2c: DisplayI2c) -> Option<DisplayDriver> {
+    None
+}
+
+// ── Board init ─────────────────────────────────────────────────────
+
+pub struct Board {
+    p: embassy_rp::Peripherals,
+}
+
+impl LoRaBoard for Board {
+    const NAME: &'static str = "Waveshare RP2040-LoRa";
+    const TX_POWER_RANGE: (i8, i8) = (-9, 22);
+
+    type RadioParts = RadioParts;
+    type CommParts = UsbParts;
+    type DisplayParts = DisplayParts;
+    type DisplayDriver = DisplayDriver;
+    type LedDriver = LedDriver;
+
+    fn init() -> Self {
+        let p = embassy_rp::init(Default::default());
+        Self { p }
+    }
+
+    fn mac_address() -> [u8; 6] {
+        mcu::mac_address()
+    }
+
+    fn into_parts(self) -> BoardParts<RadioParts, UsbParts, DisplayParts, LedDriver> {
+        let p = self.p;
+
+        // ── MAC address from flash unique ID ───────────────────
+        let mut flash = embassy_rp::flash::Flash::<_, _, { 2 * 1024 * 1024 }>::new_blocking(p.FLASH);
+        mcu::init_mac(&mut flash);
+
+        // ── SPI1 bus for SX1262 ────────────────────────────────
+        //
+        // Pin mapping from Meshtastic & MeshCore firmware (confirmed working):
+        //   SCK = GPIO14, MOSI = GPIO15, MISO = GPIO24, CS = GPIO13
+        //
+        // The Waveshare wiki (https://www.waveshare.com/wiki/RP2040-LoRa) lists
+        // different pins (MOSI=11, MISO=8, CS=9) but those don't work in practice.
+        // Both sets are valid SPI1 alternate functions on RP2040.
+        let mut spi_cfg = spi::Config::default();
+        spi_cfg.frequency = 1_000_000;
+        let spi = spi::Spi::new(
+            p.SPI1, p.PIN_14, p.PIN_15, p.PIN_24, p.DMA_CH0, p.DMA_CH1, spi_cfg,
+        );
+        let spi_bus = mcu::share_spi_bus(spi);
+
+        let nss = Output::new(p.PIN_13, Level::High);
+        let spi_device = SpiDevice::new(spi_bus, nss);
+
+        let reset = Output::new(p.PIN_23, Level::High);
+        let dio1 = Input::new(p.PIN_16, Pull::Down);
+        let busy = Input::new(p.PIN_18, Pull::Down);
+
+        // PE4259 antenna switch: GPIO17 = !CTRL (rx_enable).
+        // DIO2 is internally wired as CTRL (tx_enable), managed by the SX1262.
+        let rx_enable = Output::new(p.PIN_17, Level::Low);
+        let iv = GenericSx126xInterfaceVariant::new(reset, dio1, busy, Some(rx_enable), None)
+            .expect("SX1262 interface init");
+
+        let sx_config = sx126x::Config {
+            chip: Sx1262,
+            tcxo_ctrl: None, // No TCXO on this board
+            use_dcdc: true,
+            rx_boost: false,
+        };
+
+        let radio = RadioParts {
+            driver: Sx126x::new(spi_device, iv, sx_config),
+            delay: Delay,
+        };
+
+        // ── USB ────────────────────────────────────────────────
+        let host = UsbParts {
+            driver: embassy_rp::usb::Driver::new(p.USB, mcu::Irqs),
+        };
+
+        // ── LED on GPIO25 ──────────────────────────────────────
+        let led_pin = Output::new(p.PIN_25, Level::Low);
+        let led = crate::driver::simple_led::SimpleLed(led_pin);
+
+        BoardParts {
+            radio,
+            host,
+            display: None,
+            led,
+            mac: Self::mac_address(),
+        }
+    }
+}