Fixes #1643 - Thanks to projectkk2glider for his help on this one

Conflicts:
	radio/src/lua.cpp
	radio/src/opentx.cpp

radio/src/tasks_arm.cpp

@@ -99,6 +99,13 @@ uint32_t stack_free(uint32_t tid)
       stack = audioStack;
       size = AUDIO_STACK_SIZE;
       break;
+#if defined(PCBTARANIS)
+    case 255:
+      // main stack
+      stack = (OS_STK *)&_main_stack_start;
+      size = ((unsigned char *)&_estack - (unsigned char *)&_main_stack_start) / 4;
+      break;
+#endif
     default:
       return 0;
   }
@@ -107,7 +114,7 @@ uint32_t stack_free(uint32_t tid)
   for (; i<size; i++)
     if (stack[i] != 0x55555555)
       break;
-  return i;
+  return i*4;
 }
 
 void mixerTask(void * pdata)