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/bbb_cape/src/bbb/crc.h b/bbb_cape/src/bbb/crc.h
index b353efc..cb0bf63 100644
--- a/bbb_cape/src/bbb/crc.h
+++ b/bbb_cape/src/bbb/crc.h
@@ -1,6 +1,7 @@
 #ifndef BBB_CRC_H_
 #define BBB_CRC_H_
 
+#include <cstring>
 #include <stdint.h>
 
 namespace cape {