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/hal/mod.rs

@@ -8,3 +8,6 @@ pub mod esp32s3;
 
 #[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1"))]
 pub mod nrf52840;
+
+#[cfg(feature = "waveshare_rp2040_lora")]
+pub mod rp2040;

src/hal/rp2040.rs

@@ -0,0 +1,76 @@
+//! RP2040 MCU initialization primitives.
+//!
+//! Provides shared boilerplate for RP2040 boards: interrupt bindings,
+//! type aliases, StaticCell bus wrapping, USB init, and MAC address reading.
+
+use embassy_rp::bind_interrupts;
+use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+use static_cell::StaticCell;
+
+// ── Interrupts (shared by all RP2040 boards) ──────────────────────
+
+bind_interrupts!(pub struct Irqs {
+    USBCTRL_IRQ => embassy_rp::usb::InterruptHandler<embassy_rp::peripherals::USB>;
+});
+
+// ── MCU-level types ────────────────────────────────────────────────
+
+pub type SpiBus = embassy_rp::spi::Spi<'static, embassy_rp::peripherals::SPI1, embassy_rp::spi::Async>;
+pub type UsbRpDriver = embassy_rp::usb::Driver<'static, embassy_rp::peripherals::USB>;
+
+// ── SPI bus sharing ────────────────────────────────────────────────
+
+/// Wrap an initialized SPI peripheral in a shared bus (StaticCell + Mutex).
+pub fn share_spi_bus(
+    spi: SpiBus,
+) -> &'static embassy_sync::mutex::Mutex<NoopRawMutex, SpiBus> {
+    static SPI_BUS: StaticCell<embassy_sync::mutex::Mutex<NoopRawMutex, SpiBus>> =
+        StaticCell::new();
+    SPI_BUS.init(embassy_sync::mutex::Mutex::new(spi))
+}
+
+// ── MAC address ────────────────────────────────────────────────────
+
+/// Store for the MAC address derived from flash unique ID.
+/// Populated once during `init_mac()`, read by `mac_address()`.
+static MAC: StaticCell<[u8; 6]> = StaticCell::new();
+static mut MAC_REF: Option<&'static [u8; 6]> = None;
+
+/// Read the flash chip's unique ID and derive a 6-byte MAC address.
+///
+/// Must be called once during board init before `mac_address()`.
+pub fn init_mac<const FLASH_SIZE: usize>(
+    flash: &mut embassy_rp::flash::Flash<'_, embassy_rp::peripherals::FLASH, embassy_rp::flash::Blocking, FLASH_SIZE>,
+) {
+    let mut uid = [0u8; 8];
+    flash.blocking_unique_id(&mut uid).expect("flash unique ID");
+
+    // XOR the first 2 bytes into bytes 2..8 to fold 8 bytes into 6,
+    // then set the locally-administered bit (byte 0, bit 1).
+    let mac = [
+        uid[2] ^ uid[0] | 0x02,
+        uid[3] ^ uid[1],
+        uid[4],
+        uid[5],
+        uid[6],
+        uid[7],
+    ];
+
+    let stored = MAC.init(mac);
+    // SAFETY: Written once during single-threaded init, before the executor
+    // starts. All subsequent reads are from &'static reference.
+    unsafe {
+        MAC_REF = Some(stored);
+    }
+}
+
+/// Return the MAC address derived from the flash unique ID.
+///
+/// Panics if `init_mac()` has not been called.
+pub fn mac_address() -> [u8; 6] {
+    // SAFETY: MAC_REF is set once during init before the executor starts,
+    // and only read (never written) after that.
+    unsafe {
+        *MAC_REF.expect("init_mac() must be called before mac_address()")
+    }
+}