Do not allocate memory for LUA Rx fifo from the USB ISR

Rather use explicit allocation when the serial is set as the on-demand method that was used previously.

radio/src/lua/api_general.cpp

@@ -103,13 +103,25 @@ static int luaRxFifoGetByte(void*, uint8_t* data)
   return luaRxFifo->pop(*data);
 }
 
-void luaReceiveData(uint8_t* buf, uint32_t len)
+void luaAllocRxFifo()
 {
   if (!luaRxFifo) {
-    luaRxFifo = new Fifo<uint8_t, LUA_FIFO_SIZE>();
+    auto fifo = new Fifo<uint8_t, LUA_FIFO_SIZE>();
+    luaRxFifo = fifo;
     luaSetGetSerialByte(nullptr, luaRxFifoGetByte);
   }
-    
+}
+
+void luaFreeRxFifo()
+{
+  auto fifo = luaRxFifo;
+  luaSetGetSerialByte(nullptr, nullptr);
+  luaRxFifo = nullptr;
+  delete(fifo);
+}
+
+void luaReceiveData(uint8_t* buf, uint32_t len)
+{
   if (luaRxFifo) {
     while(len--) luaRxFifo->push(*buf++);
   }

radio/src/lua/lua_api.h

@@ -50,7 +50,10 @@ extern "C" {
 
 // LUA serial connection
 #define LUA_FIFO_SIZE 256
+void luaAllocRxFifo();
+void luaFreeRxFifo();
 void luaReceiveData(uint8_t* buf, uint32_t len);
+
 void luaSetSendCb(void* ctx, void (*cb)(void*, uint8_t));
 void luaSetGetSerialByte(void* ctx, int (*fct)(void*, uint8_t*));
 

radio/src/serial.cpp

@@ -148,7 +148,10 @@ static void serialSetCallBacks(int mode, void* ctx, const etx_serial_port_t* por
     if (getByte) {
       luaSetGetSerialByte(ctx, getByte);
     } else if (setRxCb) {
+      luaAllocRxFifo();
       setRxCb(ctx, luaReceiveData);
+    } else {
+      luaFreeRxFifo();
     }
     break;
 #endif