Get a basic USB device working

It enumerates, takes its address, gets configured, and then Linux kind
of gives up because it has no endpoints.

Change-Id: I01f75acee419b585e455f428ee45bcd37f0ce189
diff --git a/tools/environments.bzl b/tools/environments.bzl
new file mode 100644
index 0000000..68e78de
--- /dev/null
+++ b/tools/environments.bzl
@@ -0,0 +1,5 @@
+# Contains some helpers for working with environments.
+
+mcu_cpus = [
+  "@//tools:cortex-m4f",
+]