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;