Flush serial buffer after heating

Some hosts like Pronterface use continuous M105 commands to poll temperature, this causes a software halt when the heating via M109 or M190 is finished. Flushing the rx buffer fixes this.

As reported in #15, thanks to @rug156 for the feedback!

Marlin/Marlin_main.cpp

@@ -8551,6 +8551,10 @@ inline void gcode_M109() {
   #if DISABLED(BUSY_WHILE_HEATING)
     KEEPALIVE_STATE(IN_HANDLER);
   #endif
+  
+  // flush the serial buffer after heating to prevent lockup by m105
+  flush_and_request_resend();
+
 }
 
 #if HAS_HEATED_BED