Feature/mstp extended frames (#529)

* added MSTP extended frames to bacnet/datalink/mstp.c module. Thank you, Simon!

* auto-size some FIFO buffers for MSTP

* add COBS library to MSTP builds

---------

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

ports/stm32f10x/rs485.c

@@ -30,12 +30,13 @@
 #include "hardware.h"
 #include "bacnet/basic/sys/mstimer.h"
 #include "bacnet/bits.h"
+#include "bacnet/datalink/dlmstp.h"
 #include "bacnet/basic/sys/fifo.h"
 #include "led.h"
 #include "rs485.h"
 
 /* buffer for storing received bytes - size must be power of two */
-static uint8_t Receive_Buffer_Data[512];
+static uint8_t Receive_Buffer_Data[NEXT_POWER_OF_2(DLMSTP_MPDU_MAX)];
 static FIFO_BUFFER Receive_Buffer;
 /* amount of silence on the wire */
 static struct mstimer Silence_Timer;