AP_HAL: Add UART option to not override streamrates

libraries/AP_HAL/UARTDriver.h

@@ -9,6 +9,10 @@ class ExpandingString;
 
 /* Pure virtual UARTDriver class */
 class AP_HAL::UARTDriver : public AP_HAL::BetterStream {
+private:
+    // option bits for port
+    uint16_t _last_options;
+
 public:
     UARTDriver() {}
     /* Do not allow copies */
@@ -61,8 +65,8 @@ public:
     virtual int16_t read_locked(uint32_t key) { return -1; }
     
     // control optional features
-    virtual bool set_options(uint16_t options) { return options==0; }
-    virtual uint8_t get_options(void) const { return 0; }
+    virtual bool set_options(uint16_t options) { _last_options = options; return options==0; }
+    virtual uint16_t get_options(void) const { return _last_options; }
 
     enum {
         OPTION_RXINV              = (1U<<0),  // invert RX line
@@ -77,6 +81,7 @@ public:
         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
+        OPTION_NOSTREAMOVERRIDE   = (1U<<12), // don't allow GCS to override streamrates
     };
 
     enum flow_control {