AP_HAL: added serial option for disabling FIFO on uarts

libraries/AP_HAL/UARTDriver.h

@@ -74,6 +74,7 @@ public:
         OPTION_NODMA_RX           = (1U<<8), // don't use DMA for RX
         OPTION_NODMA_TX           = (1U<<9), // don't use DMA for TX
         OPTION_MAVLINK_NO_FORWARD = (1U<<10), // don't forward MAVLink data to or from this device
+        OPTION_NOFIFO             = (1U<<11), // disable hardware FIFO
     };
 
     enum flow_control {