Get some basic Teensy code working

It prints some stuff over the USB serial port and turns the on-board LED
on. Also tweak some of the existing code so it works with better
compiler options for what we're doing.

Change-Id: Ifcbd36f9eaca47a86f7a97e8c458c64f9b3df3ab
diff --git a/tools/BUILD b/tools/BUILD
index 8926d08..b8a4f1e 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -20,6 +20,10 @@
   values = {'cpu': 'roborio'},
 )
 config_setting(
+  name = 'cpu_cortex_m4f',
+  values = {'cpu': 'cortex-m4f'},
+)
+config_setting(
   name = 'cpu_armhf',
   values = {'cpu': 'armhf-debian'},
 )