Add basic rpi2040 blink demo app

Phew, things build!  Lets check in how far we got.

Change-Id: Id82e2f4c73eed6620b2b4661726f931e79a6b0c1
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/tools/platforms/BUILD b/tools/platforms/BUILD
index 4aa3db3..4df177c 100644
--- a/tools/platforms/BUILD
+++ b/tools/platforms/BUILD
@@ -47,4 +47,13 @@
     ],
 )
 
+platform(
+    name = "rp2040",
+    constraint_values = [
+        "@platforms//os:none",
+        "//tools/platforms/hardware:cortex_m0plus",
+        "//tools/platforms/go:lacks_support",
+    ],
+)
+
 # TODO(phil): Create something for "cortex-m4f-k22" builds.