(theoretically) got encoders working

Everything up to sending all 8 encoder values should work now (including
the packet format + header information, bootloader (fixed some bugs in
the UART code for that), etc).
diff --git a/bbb_cape/src/cape/main.c b/bbb_cape/src/cape/main.c
index 02f8223..32b87da 100644
--- a/bbb_cape/src/cape/main.c
+++ b/bbb_cape/src/cape/main.c
@@ -1,6 +1,6 @@
 #include <STM32F2XX.h>
 
-#include "cape/bootloader_handoff.h"
+#include "cape/fill_packet.h"
 
 // The startup asm code defines this to the start of our exception vector table.
 extern uint32_t _vectors;
@@ -9,4 +9,6 @@
   // Change the vector table offset to use our vector table instead of the
   // bootloader's.
   SCB->VTOR = (uint32_t)&_vectors;
+
+  fill_packet_start();
 }