Finished initial uart code, it compiles.

The next task is writting unit tests for it.

Also fixed some minor issues in Brian's code.
Finally, fixed a minor problem in download_externals.sh
that was causing things not to build properly.
diff --git a/frc971/atom_code/atom_code.gyp b/frc971/atom_code/atom_code.gyp
index 780ca23..a004c4d 100644
--- a/frc971/atom_code/atom_code.gyp
+++ b/frc971/atom_code/atom_code.gyp
@@ -25,6 +25,7 @@
         #'camera/camera.gyp:frc971',
         '../../gyro_board/src/libusb-driver/libusb-driver.gyp:get',
         '../input/input.gyp:gyro_sensor_receiver',
+        '<(DEPTH)/bbb_cape/src/bbb/bbb.gyp:uart_receiver',
       ],
       'copies': [
         {