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