targets: add support for Elecrow Pico rp2040 W5 boards (#4705)

https://www.elecrow.com/pico-w5-microcontroller-development-boards-rp2040-microcontroller-board-support-wifi-2-4ghz-5ghz-bluetooth5.html

Just the basics to get several test to work (blinky1, flash).

This board has an rtl8720d Wifi/bluetooth chip wired to UART1, but the
rtl8720d firmware installed is the AT cmd set, not the RPC interface
used by the rtl8720dn driver, so no wifi support at the moment.

targets/elecrow-rp2040.json

@@ -0,0 +1,12 @@
+{
+    "inherits": ["rp2040"],
+    "build-tags": ["elecrow_rp2040"],
+    "serial-port": ["2e8a:000a"],
+    "default-stack-size": 8192,
+    "ldflags": [
+        "--defsym=__flash_size=8M"
+    ],
+    "extra-files": [
+        "targets/pico-boot-stage2.S"
+    ]
+}