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.

.cargo/config.toml

@@ -11,3 +11,10 @@ rustflags = [
     "-C", "link-arg=-Tdefmt.x",
     "-C", "opt-level=z",
 ]
+
+[target.thumbv6m-none-eabi]
+rustflags = [
+    "-C", "link-arg=-Tlink.x",
+    "-C", "link-arg=-Tdefmt.x",
+    "-C", "opt-level=z",
+]