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/platform_mappings b/platform_mappings
index 04265dd..718ca72 100644
--- a/platform_mappings
+++ b/platform_mappings
@@ -12,6 +12,9 @@
   //tools/platforms:cortex_m4f
     --cpu=cortex-m4f
 
+  //tools/platforms:rp2040
+    --cpu=rp2040
+
 flags:
   --cpu=k8
     //tools/platforms:linux_x86
@@ -24,3 +27,6 @@
 
   --cpu=cortex-m4f
     //tools/platforms:cortex_m4f
+
+  --cpu=rp2040
+    //tools/platforms:rp2040