add the dma interface code

This is mostly copied from what Austin wrote.

Change-Id: I06a5f2323ec2d39ca6f1a3eaac6f4d129de84a3c
diff --git a/aos/externals/forwpilib/README b/aos/externals/forwpilib/README
new file mode 100644
index 0000000..d7ecb6c
--- /dev/null
+++ b/aos/externals/forwpilib/README
@@ -0,0 +1,2 @@
+This directory contains files that we plan to eventually push to upstream
+WPILib but haven't yet.