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/.bazelrc b/.bazelrc
index 8290da4..53f34e8 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -22,6 +22,7 @@
 build:roborio --platforms=//tools/platforms:linux_roborio
 build:armhf-debian --platforms=//tools/platforms:linux_armhf
 build:cortex-m4f --platforms=//tools/platforms:cortex_m4f
+build:rp2040 --platforms=//tools/platforms:rp2040
 
 # Without this, we end up rebuilding from scratch every time we change compilers.  This is needed to make --cpu work (even though it shouldn't be used).
 build --crosstool_top=@//tools/cpp:toolchain --host_crosstool_top=@//tools/cpp:toolchain