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/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*));