Updating accgyro files so they do not include "board.h". It is now

clear what all accgyro drivers need to compile and what was
unnecessarily included before.

src/drivers/serial_common.h

@@ -7,6 +7,8 @@ typedef enum portMode_t {
     MODE_SBUS = 1 << 2,
 } portMode_t;
 
+typedef void (* serialReceiveCallbackPtr)(uint16_t data);   // used by serial drivers to return frames to app
+
 typedef struct serialPort {
     
     const struct serialPortVTable *vTable;