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>

src/bacnet/datalink/dlenv.c

@@ -483,6 +483,8 @@ void dlenv_maintenance_timer(uint16_t elapsed_seconds)
             BBMD_Timer_Seconds = (uint16_t)BBMD_TTL_Seconds;
         }
     }
+#else
+    (void)elapsed_seconds;
 #endif
 }