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.

ld/rp2040-memory.x

@@ -0,0 +1,24 @@
+/* RP2040 memory layout — UF2 bootloader (ROM boot2)
+ *
+ *   0x10000000 – 0x10000100   boot2 (second-stage bootloader, 256 bytes)
+ *   0x10000100 – 0x10200000   Application  <- we live here
+ *
+ * Flash: 2MB (W25Q16JV on Waveshare RP2040-LoRa)
+ * RAM: 264KB SRAM (two 256KB banks + 2x 4KB scratch)
+ */
+
+MEMORY
+{
+  BOOT2 : ORIGIN = 0x10000000, LENGTH = 0x100
+  FLASH : ORIGIN = 0x10000100, LENGTH = 2048K - 0x100
+  RAM   : ORIGIN = 0x20000000, LENGTH = 264K
+}
+
+EXTERN(BOOT2_FIRMWARE)
+
+SECTIONS {
+    .boot2 ORIGIN(BOOT2) :
+    {
+        KEEP(*(.boot2));
+    } > BOOT2
+} INSERT BEFORE .text;