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/main.rs

@@ -41,7 +41,7 @@ use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel, Sta
 
 #[cfg(not(test))]
 cfg_if::cfg_if! {
-    if #[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1"))] {
+    if #[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1", feature = "waveshare_rp2040_lora"))] {
         use defmt_rtt as _;
         use panic_probe as _;
     } else if #[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart", feature = "heltec_v4"))] {
@@ -61,7 +61,7 @@ static DISPLAY_COMMANDS: DisplayCommandChannel = DisplayCommandChannel::new();
 
 #[cfg(not(test))]
 cfg_if::cfg_if! {
-    if #[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1"))] {
+    if #[cfg(any(feature = "rak_wisblock_4631", feature = "wio_tracker_l1", feature = "waveshare_rp2040_lora"))] {
         #[embassy_executor::main]
         async fn main(spawner: Spawner) {
             run(spawner).await;